An Impressive Impact on Well Productivity by Hydrochloric Acidizing of Vuggy Carbonate …


It is a fact that a acidizing opening of a carbonate is directly compared to a growth of upsurge channels in a rock. As Xiong (1994) stated, when poison is injected into a carbonate, a poison flows preferentially into a rarely permeable regions, namely a vast pores, vugs, or healthy fractures. The fast retraction of a settlement element enlarges these initial upsurge paths; so a poison shortly forms large, rarely conductive upsurge channels called wormholes. Wang (1993) reported that vast pore sizes should be benefaction in a pristine stone so that wormholing can be initiated. Moreover, if all pores in a arrangement are small, poison will disintegrate a stone regularly and wormholes will not form until some of a pores have reached a vicious size. Based on a work of Wang (1993), if vast pore sizes are accessible in sum rock, afterwards a initial settlement for a vast outcome of acidizing is granted. The vast boost of capability after acidizing diagnosis was initial complicated by Clason, 1935. He initial suggested that crevices (fractures) contingency be present, and usually by a boost of a crevices and/or dismissal of drilling liquid or other deposits from crevices or fractures could vast increases in capability be explained. Sharaf (2000) ascribed a astonishing high prolongation rates after acidizing diagnosis to opening healthy fractures. Harris (1966) introduced factors of pressure, temperature, poison upsurge velocity, poison concentration, form of acid, arrangement composition, and by-product arrangement as a categorical parameters inspiring acidizing performance. Nierode (1971) combined surface-area-to-volume ratio that is a aspect of stone that comes in hit with a given volume of acid. 

Iran has a really enlarged story as an oil producing country, following a initial blurb petroleum find in Masjid Soleyman in 1908. Normally, settlement acidizing is conducted after a good has been drilled, put into production, and when prolongation declined to say production. Accordingly, to weigh an acidizing pursuit some factors such as capability index, skin factor, vigour dump around a good gimlet and upsurge rate (before and after kick job) are achieved in Iranian oilfields. The story of settlement acidizing in some of a southern Iranian oilfields has been introduced Zoveidavianpoor (2010) to that a reader can refer. He evaluated a settlement acidizing in a south of Iran and examine that a infancy of these acidizing operations have emasculate outcome. However, there is no justification of any crevices (fractures) in many of a Iranian fields. Adding to these factors, formed on opposite acidizing practice in Iranian carbonate oil fields, during a consistent thoroughness of acid, porosity placement settlement in carbonate rocks can make a poignant difference.

In this paper, unsentimental acidizing in a standard DV oil margin located in a southwest of Iran and a considerable alleviation on prolongation is presented. To know a resource of poison in this field, a arrangement stone is analyzed on opposite beam and profitable comforts of a stone are introduced. An acidizing laboratory exam is also carried out on arrangement core samples to know a acidizing performance.

۲٫ Results and discussion

۲٫۱٫ Reservoir characteristics

The complicated oil field, called DV, is one of a extrinsic carbonate reservoirs in a southwest of Iran with a north-south elongation. This peaceful exquisite anticlinal structure is about 10 km far-reaching and 25 km long. The producing interlude consists of 3 distant layers, namely A, B, and C, that are pristine limestone with a sum firmness of about 500 m. The DV fountainhead is during a abyss of some-more than 4000 m and during a heat of about 150 °C. The initial fountainhead vigour is 9000 psi and contains light oil with an API of 40 degree. The oil prolongation is started with 3500 STB during year 2000 with one good and building adult gradually to 160,000 STB during a commencement of year 2010 with 22 producing wells. The compared gas reinjection is kicked off one year after during a aspect vigour of 7500 psi.

Undoubtedly, a special outcome of poison on this fountainhead is compared to a porosity placement and stone typing, so a fountainhead characterization is presented in a far-reaching operation from micro beam to petrophysical record scales. The many widespread facies are wackstone/packstone, peloidal packstone, ooidal grainstone, and packstone/grainstone. These comforts are famous from scanning nucleus microscope images shown in Figures 1-4. These images are skinny territory images entrance from core cut samples in one well. It is formidable to envision a acidizing outcome from these images, though they can be used to impersonate a reservoir. The parallel and straight distributions of such facies are tranquil by a high magnitude sea turn variations generating built transgressive/regressive sedimentary cycles.


Figure 1

Packstone/grainstone and a categorical pore forms are represented by intergranular and vugs.


Figure 2

Wackstone/packstone, with benthic foram.



Figure 3

Ooidal grainstone, a primary porosity is filled by calcite cements.


Figure 4

Well-sorted peliodal packstone; a primary porosity is filled by calcite cements

The depositional sourroundings of DV fountainhead is of a ramp form carbonate platform. The facies belts of inner, outer, and center ramp can be recognized. The center ramp is characterized as a carbonate rave that could be compared with prograding facies and shoals with primary porosity such as intergranular and horizon porosity. The center and outdoor ramp facies are represented by wackstone and mudstone with primary bad fountainhead peculiarity properties. The categorical lithology of this fountainhead is limestone interbedded with mudstone to shaly limestone. During geological time, dual vicious retraction events have occurred: early diagenetic building intercrystalline/micro porosity, molds and vugs adult to 1 mm in distance in a top half of a fountainhead (layer A and B), and paleokarst formulating delegate pores in a form of vast molds and vugs adult to 2 cm in distance with resolution lengthened comforts and resolution pipes many widespread in a revoke half of a fountainhead (Layer C).

۲٫۲٫ Reservoir porosity analysis

In sequence to have samples of arrangement rock, coring operation is achieved in estimation wells. Figure 5 shows an X-ray tomography picture indicating a placement of delegate porosities (dark areas) in 6 slices of a plug. This is an denote of spatial delegate porosity placement also in a formation. As shown in this figure, delegate pores are contentment and good connected along a core plug.


Figure 5

X-ray tomography images.

Ditch slicing samples in vuggy intervals are also analyzed regulating an visual microscope. Figure 6 shows a pore network characteristics of cuttings in DV field. As detected, there are some microfractures (yellow arrow), that can bond a moldic porosity (red arrow) to other sections of a media. The mcirofracture is not utterly open, though substantially exposing to adequate poison can make a passage between moldic porosities.


Figure 6

The porosity is comparatively high and done adult by molds (red arrow), and somewhat healed microfracture (yellow arrow).

Moreover, Schlumberger has introduced a new proceed of non-static firmness record (VDL) to implement borehole electrical images in a research of carbonate fountainhead porosity complement (Newsberry, 1996). Through this technique, porosity placement and apportion of vugs/molds fragment around a wellbore during any abyss can be obtained. The resistivity information from electrical images are remade into porosity map of a borehole after their calibration with a shoal resistivity and record porosity (preferably effective porosity). At any specified sampling rate (generally 0.1 in. for oomoldic porosity complement or 0.3 in. or bigger for formations carrying vast distance vugs or molds), porosity placement histograms are computed. Figure 7 shows opposite porosity distributions in homogenous and extrinsic carbonates. As shown in Figure 7 (a), for homogenous intervals, a porosity placement has a bell figure frequency, that is called unimodal placement (homogenous carbonate). In vuggy carbonates, rarely lopsided unimodal or bimodal placement of porosity is observed. While in many extrinsic carbonates where cementation, primary porosity, and vuggy porosity are present, rarely lopsided and broad, or bimodal/trimodal porosity placement might be celebrated (Figure 7 (b and c)). On such histograms, a points from a high porosity ends paint leached pores (vugs or molds) and detonate fractions of porosity, given a points from a low porosity finish go to a unenlightened or cemented areas of a horde rock. The area underneath a high porosity tails of porosity histograms gives a apportion of delegate porosity.




Figure 7

Typical FMI/FMS porosity histograms arrangement porosity placement in homogenous and extrinsic carbonates; unimodal placement is found in homogenous carbonates (a); bimodal to trimodal distributions are found in extrinsic carbonates (b and c).

Figure 8 displays porosity histograms, non-static firmness record (VDL) along with FMS images. The porosity research suggested a participation of delegate porosity in many territory of a reservoir. The porosity histogram shows that a top territory of a interlude has a broad/skewed porosity placement as a pointer of some-more delegate porosity, that is especially stoical of vugs/molds. In a revoke interlude of Figure 7, a heterogeneity seems to be less, though an uneven porosity placement is still visible. The delegate pores are also manifest on a FMS display. The FMS images suggested varying volume of heterogeneity in a form of conductive and resistive (dense) areas opposite a whole interval. The conductive heterogeneities are due to porous areas (i.e. rags of intergranular and intercrystalline porosity, moldic/ vuggy porosity and, brief discontinuous open fractures) of opposite sizes, shapes, and conductivities. The resistive heterogeneities are due to unenlightened cemented areas of revoke or 0 porosity. As can be seen, no justification of open fractures carrying a continual conductive snippet could be found in record data. The same formula were achieved when questioning good exam and seismic traces. Delving into a prolongation logging information showed that delegate porosity intervals have a many grant in production.


Figure 8

FMS porosity histograms and VDL (variable-density-log) arrangement over a territory arrangement extrinsic (broad porosity range) and uneven porosity distribution.

Ghafoori (2009) went by a petrophysical record of this fountainhead and resolved that there is a honeycomb settlement of delegate pores/channels in a unenlightened horde stone of this carbonate reservoir. They impute to a prolongation logging information as a fact that a retraction eventuality caused by paleokarst has a certain outcome on fountainhead productivity. Most sections of covering C in DV margin have extrinsic trimodal porosity (as shown in Figure 8); covering B is sincerely influenced with this distribution, and covering A has a slightest porosity heterogeneity according to Schlumberger record interpretation.

۲٫۳٫ Method of unsentimental acidizing in DV oil field

Basically opposite techniques were used to deliver poison to a arrangement in DV oil field. Since a arrangement is low (more than 4000 meters), a function of curl tubing is unsure and all a poison is self-willed to a well. In a early acidizing jobs, sequences of poison and gelatinous ludicrous deputy (VDA) were injected; supposedly, VDA should obstruct poison to rebate permeable intervals. In a after cases though regulating VDA, diversion pumping technique was practiced; poison injection rate was augmenting in several steps, and a good conduct vigour dump was checked in any step. If a vigour was fast during a consistent injection rate, it means that a poison has mislaid a reactivity, and it is a finish of pulling poison serve into a formation. Latest jobs were achieved by usually delivering poison to a arrangement during a consistent rate in sequence to mislay a high skin evaluated from early good tests. Pumping is continued until displacing liquid fill adult a wellbore and prolongation tubing.

Acidizing debate was achieved in DV oil field, and a volume of liquid and a formula of acidizing are shown in Table 1. For any well, execution status, producing layer, firmness of a formation, evaluated skin from good test, injecting liquid per meter, final pressure, and rate and volume of capability index before and after acidizing are shown in Table 1. As can be seen, a volume of poison per scale of producing intervals are different, though comparing with categories mentioned with opposite authors, it is not a high volume acidizing (Sharaf, 2000, Halliburton, 1998, King, 1986).

Table 1

Matrix kick outline of wells in a margin studied.

* Well is circuitously injector and, due to acidizing, a GOR augmenting from 2500 scf/stb to 7000 scf/stb.

The initial pursuit was achieved on good xx5, that had not been means to furnish any oil earlier. This good is a usually good finished in covering A. Based on accurate petrophysical interpretation and MDT tests, this whole covering did not have poignant mobility. However, it was in a agreement of a building association to finish during slightest one good in a many prolific territory of this layer. As a result, a good was finished in Layer A.2. However, after perforation, good conduct vigour forsaken to 0 and no oil could be produced. As shown in Figure 9, in early Sep 2011, a initial pursuit with hydrochloride poison of 15 percent thoroughness was performed. In this operation, a arrangement is initial spotless adult by injecting 6 barrels of xylene, and afterwards 26 barrels of poison was self-willed to a formation. As tabulated in Table 1, with 1.3 barrels of poison per scale of arrangement during a limit injection rate of 0.5 barrels per minute, diversion technique was used to make certain poison is unprotected to all formation. However, as shown in Figure 9, a good prolongation softened for a brief duration with 1000 bbl/day until asphaltene plugged a good during a finish of September. After that, several asphaltene clean-ups with xylene were implemented, though a good could not furnish oil for a enlarged time before asphaltene block it again. Second acidizing was achieved after perforating a 25 scale new interlude with weaker petrophysical and capability properties. This time, 186 barrels or 4.1 barrels per scale of poison after 50 barrels of xylene was injected during limit pumping equine energy to inject during 4160 psi and 6.4 bpm a outcome was extraordinary. As shown in Figure 9, all prolongation factors rocketed up. Well conduct vigour augmenting and during 3000 psi issuing good conduct pressure, and some-more than 10,000 bbls/day oil was produced. Asphaltene problem was tackled with escalated wellhead temperature, and a good capability leapt from 0.1 to 7.5 stbd/psi.


Figure 9

Wellxx5 finished in covering A, arrangement endless improvement.

The capability alleviation was continued by acidizing wells in covering B as well. For example, good x0, as shown in Figure 10, had serious asphaltene inundate problems, and it was compulsory to perform asphaltene cleanup during slightest 3 times in a year. The good was acidized for a initial time usually before Mar 2010. As tabulated in Table 1, about 4.2 bbls/m 15% hydrochloride poison was injected, and capability was softened from 1.5 to 2.6 stbd/psi. The good was returned to prolongation during a good issuing vigour of some-more than 3000 psi and a rate of some-more than 1500 bbls/day. However, a problem of asphaltene plugging was not solved. One year later, a good was acidized for a second time with 4.7 bbls/m of poison during a aloft injection rate of 7.6 bbls/m. As shown in Figure 10, after second acidizing, a good was producing invariably for some-more than a year with a daily prolongation of some-more than 5000 bbls/day.


Figure 10

Well x0 finished in covering B; adding some-more poison resulted in aloft improvement.

In covering C, a acidizing formula were some-more conclusive. As shown in Table 1, usually in 3 wells gelatinous ludicrous deputy was utilized, but, in comparison with other jobs (without VDA), no allied alleviation was detected. Except in good x08, that is tighten to a gas injection wells, and a GOR augmenting from 2500 scf/stb to 7000 scf/stb due to acidizing, other wells had astonishing responses to acidizing. For instance, Well xxx1, as shown in Figure 11, was started producing from Jul 2009 with rebate than 2000 psi issuing good conduct vigour and about 2500 bbls/day oil production. The good was acidized in Feb 2011, and a vast prolongation encouragement was achieved. The oil prolongation leapt to 11,000 bbls/day for a issuing wellhead vigour rebate than 3000 psi. In all a other wells finished in covering C, a increment in productivity, as shown in Table 1, is apparent and in approach attribute with a volume of poison per scale of formation. It seems there is no limitation; a some-more a poison was injected, a some-more a capability was achieved. If there was not any automatic restraints, we could exam a thought of some authors that carbonate acidizing treatments can be carried out during a top rate presumably though fracturing a fountainhead rock. In this box for this special field, some-more capability alleviation might be foretold. In Well x5, a good contrast was carried out before and after stimulation. Pre-stimulation skin was distributed to be 480 and post kick skin was reduced to 7. Therefore, there is some-more room for improvement.


Figure 11

Well xxx1 finished in covering C produces a outrageous volume of oil after acidizing.

However, what is a reason for a high potency of acidizing and since do 3 layers have opposite responses to acidizing? As Tiab (2004) stated, a vicious couple between geological heterogeneity and fountainhead opening is a pivotal to know a carbonate reservoir. Several studies from sedimentological deposition to little samples have been conducted to find out a special comforts of this field.

۲٫۴٫ Core kick experiment

Although it is not probable to yield a deputy core from vuggy formation, some core plugs were prepared to perform poison inundate experiment. Moreover, due to a rarely extrinsic inlet of core, it was not probable to take enlarged core plugs. First of all, collection exam was achieved on one core sample. One core was belligerent to powder and placed in a 15% HCl resolution during room temperature, and a weight of a representation was totalled continuously. The formula were compared with a pristine limestone sample, and a poignant disproportion was not observed. Therefore, it was resolved that a greeting rate of fountainhead stone was utterly identical to pristine limestone, and there was not any special combination in a mineralogy of a stone to make it different. Second, one acidizing setup was prepared to inject during a limit vigour of 2500 psi and during an windy backpressure. To start a experiment, core measure and porosity were measured. In sequence to have a in situ condition of a reservoir, a gasoil (diesel) was initial injected to obtain a permeability of any core. Afterward, poison during opposite concentrations and injection rates was unprotected to one face of a core, and a sum volume injected was monitored until poison reached a other face of a core. The formula are listed in Table 2. As suggested in this table, a examination is steady with poison concentrations of 10, 15, and 20% and during injection rates of 0.125, 0.25, and 0.5 cc/minute. In any set of a experiments, one core representation with a possess porosity and heterogeneity was utilized.

Table 2

Acid injection experiments.

*Core was jam-packed with asphaltene.

Since DV fountainhead has a problem of asphaltene plugging in some wells, a acidizing examination is achieved on a core jam-packed with asphaltene components. However, no poignant outcome on acidizing opening was observed. In all a cases, a wormhole, as shown in Figure 1 was rescued in both side of a core. Moreover, given a series of accessible cores were limited, it was not probable to repeat a experiments for some-more cases. Also, from these data, no end can be drawn since a earthy properties of a stone (mainly porosity distribution) are changing in any core. This is another special underline of vuggy carbonate stone that can't yield dual core samples with a same porosity distribution.


Figure 12

Wormholes in dual sides of a core after acidizing with 15% HCl and during a rate of 0.125 cc/min.

Besides, a core plugs paint usually a settlement territory of a specimen, and swelling a outcome of a examination to a whole producing arrangement can't be utterly accurate. However, deliberation a initial formula and high heat and vigour conditions of a field, it was motionless that a best poison thoroughness of 15% during an initial rate of 0.25 cc/minute was some-more fit than 20% or higher. This thoroughness with retarding additives should not have any risk of corrosion.

In total, effective acidizing in DV margin has augmenting a haven and resolved several prolongation challenges. First of all, after acidizing, due to poignant skin reduction, approaching margin plateau elongated. In Figure 13, a immature bend is before kick and a red bend is a post kick margin production. As seen, a acidizing alleviation of a wells resulted in an prolongation of plateau for some-more than 3 years. Second, after acidizing, low vigour dump was unprotected to a arrangement face to benefit a high rate; as a result, a molecule prolongation was reduced saving longer opening of a aspect oil diagnosis facilities. Third, when prolongation of a good increased, aspect issuing heat was increased, and hence a asphaltene plugging (or polish deposition) both in a tubing fibre and aspect comforts were reduced (Hasanvand, 2015). In Figure 13, a prophecy shows that a oil prolongation of post-acidizing falls next pre-acidizing prophecy in 2032, that is due to a fountainhead vigour drop. In box of post-acidizing, some-more oil is extracted from a reservoir, and consequently, in comparison, somewhat rebate prolongation is expected.


Figure 13

Production foresee before and after kick (from fountainhead model).

There is also a obstacle for acidizing in this special field. A good tighten to a gas injector was also acidized. The skin again reduced significantly, but, as a result, gas oil ratio augmenting dramatically. Thus, it is resolved that by acidizing high firmness vuggy carbonate, a possibility of opening trail between injector and writer is also augmented. Moreover, due to this increment, a user contingency revoke a oil rate to forestall extreme gas production.

Frick et al. (1994) resolved that a outcome of a high permeability formations would be a same as a high injection rate. In both cases, due to a growth of several wormholes along a whole section, some-more volume of poison is compulsory to dig to a certain diameter. In this complicated case, a arrangement is rarely vuggy and some-more poison is indispensable to dig deeper into a formation. The formula of a experiments shown in Table 2 reliable this statement. In a experiments, solely for a box of 10 % hydrochloride acid, when a permeability or injection rate increases, a volume of poison to strech breakthrough will also increase. In margin practices shown in Table 1, by augmenting both poison volume per scale and poison injection rate capability index was improved. The singular indicate about DV margin box is that we could not find any reduction for improvement; in other words, a some-more a poison is injected, a some-more a alleviation is achieved. It seems there is no reduction of alleviation due to a percolation of vuggy porosity. Clason (1935) believed that fractures should be benefaction for vast acidizing improvement; however, in this case, detonate is not detected. Instead, abounding vast pores like a honeycomb are rescued Ghafoori (2009). When vast pore sizes are available, as Wang (1993) stated, a initial criteria for vast outcome of acidizing is afterwards available. Since this presented box has lots of vast pores, total volume of wormholes will form if adequate poison exposed.

As a outcome of acidizing campaign, a margin plateau is elongated, and prolongation hurdles like molecule prolongation and asphaltene plugging are avoided.

۳٫ Conclusions

Hydrochloride acidizing is an typical process of capability encouragement in carbonate rock, though delegate porosity placement as a categorical cause inspiring arrangement opening should be emphasized. In DV reservoirs, a normal volume of hydrochloride poison is injected, and astonishing formula are recorded. The full margin investigate is performed, and a following conclusions are obtained:

  1. A paleo karst materialisation has distributed vast pores in DV carbonate fountainhead in such a high firmness equivalent to honeycomb, that quantifies a prolongation sharing.
  2. The specific porosity placement settlement (honeycomb pattern) caused high automatic skin during drilling on a one hand, and helped with a percolation of a hydrochloride poison on a other hand, heading to vast capability improvement.
  3. Practical formula in DV oil fountainhead uncover that by injecting a aloft volume of poison aloft good capability is achieved. Since vast pores are abundance, a bearing of some-more poison can satisfy serve wormholes.
    1. The acidizing augmenting a ultimate liberation of a margin with existent wells and enlarged a prolongation plateau. This will revoke a cost of margin development, and it is value characterizing a arrangement before finalizing margin growth plan. Furthermore, it has mitigated some prolongation hurdles like asphaltene (or wax) inundate and molecule production. This anticipating helped with shortening a cost and risk of good unplugging operations.


The authors would like to appreciate Arvandan Oil and Gas Company (AOGC)/ National Iranian Oil Company for their team-work and support to tell a formula of their superb achievements.


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