Analysis and countermeasures of drilling challenges in complex ultra-deep exploration wells in thrust structural belt with multiple salt layers

The deep and ultra-deep zones of Bokhtar oilfield in Tajikistan are the critical blocks for future exploration and development, which are imbedded between two salt layers, accompany with four reservoir and cap assemblages of middle and upper Jurassic, Lower Cretaceous, Upper Cretaceous and Paleogene. The reservoir and cap assemblages of pre-salt carbonate and salt-gypsum cap have great potential. The complicated geological structure, uncertain formation and non-developed pre-salt formations made the critical challenges for drilling operation, including: (1) The difficult casing program design caused by peripheral compression and thrust of the basin, the complicated geological structure, the large geological difference above and below the salt layer, the unfaithful prediction of multiple sets of salt layer, great formation pressure uncertainty, and many sealing points; (2) The upper salt formations are featured by developed thrust belt and large dip angle, the contradiction between deviation prevention and drilling acceleration in high steep structure is prominent, and the wellbore quality is difficult to control; (3) Due to the mud loss in upper salt layer, loss coexisted with blow out in the salt layer and inaccurate prediction of salt bottom, the drilling complexes such as mud loss, overflow and well collapse are occurred frequently; (4) Ultra-deep well depth (over 7000m), HTHP (180°C,140MPa), multiple salt layers and long open hole section made high requirements on high temperature resistance of drilling fluid, cementing slurry and downhole tools, as well as high drilling risk and cost. It is necessary to carry out the drilling difficulties investigation of ultra-deep exploration well under the consideration of pertinence, safety, applicability and economy, then formed the suitable drilling engineering plan for Bokhtar Block through the optimization of domestic and foreign advanced and mature deep and ultra-deep well technologies, which is used to provide the technical support for the first pre-salt ultra-deep exploration well in this Block to be drilled successfully.


Introduction
With the progress of oil and gas geological theory and engineering technology, global oil and gas exploration has been moving towards deep and ancient strata, and deep oil and gas resources have become the main growth of proved reserves in the past 10 years.The deep exploration of clastic rocks refers to the exploration depth of more than 3500m in eastern China and more than 4500m in Western China; Carbonate deep exploration generally refers to exploration depth greater than 4500m.By the end of 2018, 68 oil and gas reservoirs with depths of more than 8,000 m had been discovered worldwide [1].China has made fruitful achievements in deep and ultra deep oil and gas exploration.By the end of 2018, more than 20 deep oil fields had been discovered, with a total of 4 billion tons of Original Oil In Place (OOIP) and more than 10 deep gas fields, with a total of 5 trillion cubic meters of Original Gas In Place (OGIP).
After nearly 30 years of development, overseas oil and gas exploration business has made a number of major discoveries, which has strongly supported the substantial growth of overseas oil and gas production.The large superimposed petroliferous basins in the Middle East, Central Asia and Russia are rich in oil and gas resources, with relatively high exploration level in the middle and shallow layers, perfect development facilities and superior deep and ultra deep oil and gas geological conditions, which are the practical field and important direction of large-scale oil and gas exploration.In Central Asia and the Middle East, two sets of Paleozoic and Mesozoic oil and gas systems dominated by gypsum salt-carbonate-shelf mudstone rock are developed in the Huanxin and Paleotethys oceans, of which the burial depth of Jurassic contract area in Tajik Basin is more than 5000 meters [1].
Since the 1920s, 1881 wells have been drilled in Bokhtar block, most of them are shallow wells drilled into pre-salt formation by Former Soviet companies.CNPC Engineering Technology R&D Company Limited (CPET) has rich experiences in High Temperature and High Pressure (HTHP) deep oil and gas exploration and drilling in Tarim Piedmont structure, Sichuan Chongqing deep shale gas, Mingbulak project, right Bank of Amu Darya Project and other blocks, and has formed a series of matured and advanced drilling and completion technologies, which can be used to guide the drilling of deep gas wells in Bokhtar exploration area.

Geology description
Bokhtar block is located in the southwest of Tajikistan and the northeast of Afghan-Tajik basin, with an area of about 36,000 km 2 .It is structurally divided into "two uplifts and two depressions": Kafernigan uplift, Vakhsh depression, Obigarm uplift and Kuliyab depression.The target suprasalt well and pre-salt well are located in the south and north of Kafernigan uplift, respectively (shown in Figure 1).Affected by the collision between the Indian plate and the Eurasian plate in the Neogene Miocene, the Tajik basin developed a double-layer structure above and below salt, forming two sets of exploration strata, high steep above salt and broad below salt, as well as 4 sets of reservoir cap assemblage, which are Middle and upper Jurassic, Lower Cretaceous, Upper Cretaceous and Paleogene, the pre-salt carbonate rock and salt gypsum rock reservoir cap assemblage has great potential.The suprasalt reservoir forming assemblages are mainly Cretaceous and Paleogene source reservoir cap assemblages.There are 29 potential target traps in the Paleogene and 28 in the Cretaceous, with a post risk oil resource of 238 million tons.Due to the strong influence of tectonic movement, small and medium size oil and gas reservoirs can be formed.The pre-salt reservoir forming assemblage is dominated by Jurassic reservoir cap assemblage, with good reservoir forming conditions.Three favorable structural zones and 15 traps are developed, with a total area of 2022 km 2 .The prospective resource after risk is 1.12 trillion m 3 , which has the conditions for forming large and medium-sized gas fields.However, due to the deep burial depth (>7000m), it is hard to be exploited.The drilled formations encountered many nappes and with great stratigraphic differences.All the existed wells encountered the salt layers with different vertical depth distribution.The main sedimentary sequence is the Mesozoic Cenozoic, and the burial depth of pre-salt Jurassic carbonate reservoir is 6000-8000m.The drilling encountered formations of Kafernigan thrust fault hanging wall and footwall are Quaternary, Cretaceous, salt and Paleogene, Cretaceous, salt, middle and upper Jurassic carbonate rocks, respectively.Abnormal high pressure is widespread, and 78% of the traps have been found to have abnormal high pressure in the region.Longitudinally, Jurassic, Cretaceous and Paleogene are distributed, with pressure coefficients ranging from 1.31 to 2.29, which varies greatly.The average temperature gradient is 2.23 ° C/100m, the minimum temperature gradient is 2 ° C/100m and the maximum temperature gradient is 4 ° C/100m.The geothermal gradient in shallow layers is higher than in deep layers.The distribution range of acid gas is wide and the content varies greatly.H2S is ubiquitous in the upper Jurassic reservoirs, with the content ranging from 0.1% to 6%, CO2 is mainly distributed in the middle and lower part of upper Quaternary, with the content ranging from 0.2% to 5%.

Offset well information
According to the available data, since 1920s, 1881 Wells have been drilled in Bokhtar block, most of which are shallow Wells below 1500m, accounting for 56.35%.A total of 115 traps have been drilled.Wells with depths between 2000 and 4000m are concentrated in the west-central and southern parts of the block, and those deeper than 4000m are distributed in the north and southwest of kafernigan and the central part of Vakhsh depression.The casing program of existed wells that deeper than 5000m is mainly 4-spud structure, using two layers of technical casing to seal the multiple nappes, the 2nd spud casing was run deeper than 2000m.In which, well South Karatau 1(6101m) and well Tuyuntau 2(5652m) are located in the south of Bokhtar block, well Abdulaka 1(5009m) and well Reangn 1(4946m) are in the north.The downhole complex includes mud loss, salt water invasion, oil and gas invasion, collapse, tool sticking and so on, of which mud loss (55.5%) and well invasion (32.5%, overflow rate was up to 0.6-360 m 3 /d) are the most important complex that accounting for 88% of total downhole complex.The mud loss mainly occurred from 800m to 2000m (shown in Table 1).The downhole complex in Cretaceous and Neogene occupied 42.1% and 40.1% of the total, respectively.The drilling period of Bokhtar block is generally longer than that in China.Analysis of the available data in the Kafernigan field shows that excluding the wells with a period of more than 1400 days due to objective factors, the average period of wells with a depth of less than 4000m is more than 500 days, and the average period of wells with a depth of more than 5000m is more than 700 days (shown in Table 2).

Drilling challenges analysis
Drilling challenges in complex ultra-deep exploration well in thrust structural belt with multiple salt layers are: (1) Complex geological structure and formation pressure system lead to complex wellbore structure.The basin suffers from peripheral compression and thrust, with complex geological structure, large geological differences above and below salt [2], difficult prediction of multiple sets of salt layers, large uncertainty of formation pressure, many necessary sealing points, and difficult well structure design.The casing program designed by TOTAL is up to 9 layers.
(2) The dip angle of the upper formations is large (may reach more than 70°), so it is difficult to prevent the well deviation.The upper salt formations are featured by developed thrust belt and large dip angle, the contradiction between deviation prevention and drilling acceleration in high steep structure is prominent, and the wellbore quality is difficult to control [3,4].The deviation angles of Northern 020 @3850m, Kyzyltumshuk 025 @775m and Pushion Northern 003 @2200m are 17°30′, 20°15′ and 34°, respectively.
(3) Due to the mud loss in upper salt layer, loss coexisted with blow out in the salt layer and inaccurate prediction of salt bottom, the drilling complexes such as mud loss, overflow and well collapse are occurred frequently [5,6].The strata are repeated for many times, and there are great differences in the strata encountered during drilling.The extremely thick salt gypsum layer containing high-pressure brine is developed.According to data statistics, the number of lost circulation and well invasion occurred in Eogene and Cretaceous accounted for 93.7% and 83.5% of the total complex accidents.
(4) The designed well depth is over 7000m, downhole HTHP makes a high requirement on drilling fluids and tools.The geothermal gradient of the block is 2-4 ℃/100m, which indicated that the downhole temperature may reach up to 180 ℃.There are abnormal high pressures in the lower Cretaceous, Upper Cretaceous and Paleogene in Kuliyab, Vakhsh and Dushanbe depressions, it is predicted that the bottom hole pressure of designed well may reach up to 140MPa.The downhole HTHP makes high requirement on drilling fluid, cementing slurry and downhole tools.

Casing program optimization
As mentioned above, due to the complex geological structure and formation pressure system, combined with the analysis of downhole complex and accidents of existed well, 5 sealing points are determined (shown in Figure 2).
Firstly, the upper unconsolidated layers need to be sealed so as to set the stage for wellhead control.Secondly, based on the downhole complex analysis result of exist wells, the Pliocene contains complicated formations which are easy to cause mud loss and wellbore collapse, the technical casing should be used to seal this part.Next, all the existed production wells in this area were drilling to develop the Palaeogene reservoir above the salt layer, which means the reservoir above the salt layer has to be sealed this time to avoid of overflow and oil gas invasion.Fourthly, the developed salt gypsum layer exists in the Upper Jurassic, as well as may be embedded with high pressure salt water, all of which lead to many downhole accidents such as wellbore collapse, casing failure due to salt layer creep and high pressure salt water invasion [2,3].Lastly, refer to the developing experience of adjacent blocks, it is estimated that the pre-salt pay zone is a HTHP reservoir, it is recommended that the production liner should be applied to seal the reservoir.The biggest advantage of this design is that there are many backup measures can be implemented according to real operation situations.
Case 1: 1 layer backup casing for above salt sections.If 12-1/4" bit can not drill through the salt layer (low pressure above salt+high pressure within salt), backfill the salt layer and ream the hole to 14-3/8", run φ299mm expandable tubing to increase the formation bearing pressure, which is used to secure the normal drilling within the salt layer.
Case 2: due to the uncertain condition of pre-salt formation, reserve 2 layers of casing.If the downhole complex occurred after drill through the salt layer, φ 194mm expandable tubing is recommended to seal the complicated formation which is hanged on 9-5/8" casing, and then drill to the designed well depth with 7-1/2" bit.If the downhole complex occurred again during the 7-1/2" drilling process, it can use 5-7/8" bit to drill to the designed well depth, and hang 5" integral joint casing, tie back 7" casing (if needed).
Case 3: reserve 1 layer casing for salt layer.If 9-5/8" can not seal the salt layer completely, run 7" casing prematurely, use 5-7/8" bit to drill to the designed well depth, and hang 5" integral joint casing, tie back 7" casing.During 5-7/8" drilling, if there is downhole complex, 4" bit drill to the designed depth with open hole completion.

Vertical drilling technology
According to the technical investigation at home and abroad, compared with conventional drilling, the adoption of vertical drilling technology can increase the Rate Of Penetration (ROP) by 5-6 times, and the well deviation is controlled within 1° [4,5], which can realize casing protection and drilling acceleration (shown in Table 3).BH-VDT developed by Bohai drilling company has been applied in Tarim and Qinghai oilfields, and the well deviation is controlled within 0.5°.The foreign products such as ZBE5000 has been applied in deep well KTB (3260-6760m), the deviation angle was controlled within 0.5° [4,6], and the Power-V developed by Schlumberger has been widely used in Tarim oilfield, the working time of single trip was more than 100h and the maximum deviation angle was less than 1°, another famous vertical drilling system VertiTrack supplied the service in Foothills area of Canada, in where the deviation angle was controlled smaller than 0.6° .

Lost prevention and control technology
The most widely used drilling fluid systems in China are oil based drilling fluid with density of 2.6g/cm 3 , temperature resistance up to 220 ℃ and salt water pollution resistance of 30%, water based drilling fluid with density of 2.8g/cm 3 , temperature resistance of 210 ℃ and organic salt based drilling fluid with density up to 2.85g/cm 3 , temperature resistance up to 210 ℃, respectively (shown in Figure .4) [7].The performance of foreign similar products is oil based mud with density of 2.6g/cm 3 , temperature resistance of 260 ℃, water based mud with density of 2.5g/cm 3 , temperature resistance of 240 ℃, and synthetic based drilling fluid with density of 2.8g/cm 3 , temperature resistance of 230 ℃, respectively [7,8].

Performance in reservoir formation
In terms of cementing slurry technology, the high temperature and salt resistant cementing slurry system based on large temperature difference retarder is mainly adopted in China, with a density of 2.60g/cm 3 and temperature resistance of 200 ℃, which can meet the cementing requirements of large temperature difference of 80-120 ℃ , and the strength of cement sheath reaches 10MPa.The performance of foreign deep well cementing slurry is better than that in China.Taking Schlumberger FlexSTONE HT high temperature flexible slurry as an example, the density is 2.88g/cm 3 , the temperature resistance is 260℃, and the maximum strength of cement sheath can reach 20MPa [9].The cementing tools, such as liner hanger, stage collar and outside tubing packer, the maximum pressure and temperature resistances of domestic and foreign products are 70 MPa/150℃and 70 MPa/204℃, 35 MPa/150℃ and 40 MPa/200℃, 25 MPa/150℃ and 105 MPa/260℃, respectively [10].

Drilling acceleration technology (drilling bit technology)
PDC bits with special cutting tooth structure and anti-abrasive and anti-impact materials are preferred both in China and abroad, and the deep and ultra deep well bit technology series based on PDC bits are integrated to meet the ROP increase demand of deep and ultra deep wells [11].In China, special cutting tooth structures such as non-plane teeth are mainly used (shown in Figure . 5), the impact resistance can be increased by more than 10 times compared with conventional PDC bits.Through investigation and application, the ROP is increased by 20%-250% compared with adjacent wells, and the footage of single bit is increased by 30%-500%.The foreign companies have different types of cutting tooth bits, such as Schlumberger axe-tooth bits, which can increase penetration rates by 60% compared to offset wells, the penetration depth is 20% greater than that of conventional PDC bits, and the footage of single well bits can be increased by 150%.

Conclusions
(1) There are 1881 wells drilled in Bokhtar block, most of which are shallow wells below 1500m, and deep wells below 7000m have few drilling data and many unforeseen factors.This paper carries out a casing program conceptual design based on the offset wells' information, geological characteristics of adjacent blocks and successful experiences in domestic and abroad, which provide the fundamental technical support for the first ultra deep exploration well to be drilled.
(2) Complicated ultra-deep exploration wells with multiple salt layers in thrust structural belt are mainly faced with complex wellbore structure, difficult deviation prevention and straightening, complex and frequent accidents such as downhole leakage, well invasion and well collapse, and high requirements for drilling fluid and tools.
(3) In order to provide technical support for the successful drilling of complicated ultra-deep exploration wells in multiple salt layers in thrust structural belt, it is necessary to carry out the

Figure 1 .
Figure 1.2D interpretation structural map of Bokhtar block.

Figure 4 .
Figure 4. Drilling fluid performance in salt layer and reservoir.

Table 1 .
Downhole complex statistical analysis of Bokhtar block.

Table 2 .
Partially drilling period of southwest of Kafernigan block.

Table 3 .
Comparison between domestic and foreign VD systems.