1) The marine sedimentary basins abroad are dominated by Mesozoic and Cenozoic strata, while the marine carbonate strata in China are dominated by Paleozoic strata, which are old and buried deeply. Most offshore oil and gas wells in Tarim Basin and Sichuan Basin are ultra-deep wells (Masaru Ibuka is at 6000m).
2) The Paleozoic marine sedimentary basin in China underwent the transformation of multi-stage tectonic movement. The original multi-type sedimentary basins were strongly reformed after uplifting and denudation, and then reorganized and reformed under the superposition of basins in the later period, forming a variety of reservoir types. At the same time, the later diagenetic evolution strongly transformed carbonate reservoirs, resulting in strong heterogeneity of marine carbonate reservoirs, diverse reservoir space types, and the development of pores, caves, fractures and their complexes, often accompanied by high temperature and high pressure characteristics.
3) Due to the development of some gypsum rocks near marine hydrocarbon sources and reservoirs, thermochemical sulfate reduction reaction occurs at high temperature, which is rich in acid gases such as H2S and CO2, and the two gases often coexist.
(1) Tarim basin
It is difficult to drill deep wells and ultra-deep wells in Tarim Basin.
1) Complicated geological structure and formation pressure system lead to poor accuracy of formation pressure prediction and detection, difficulties in reasonable wellbore structure design, and a series of difficulties such as reasonable drilling fluid density design, wellbore stability, leakage prevention, channeling prevention and sticking prevention.
2) The development of deep salt layer and compound salt-gypsum layer faces a series of technical difficulties in drilling salt-gypsum layer, such as well bore structure optimization design, casing strength design, drilling fluid technology in salt-gypsum layer and cementing technology.
3) The development of ultra-thick shale has brought about wellbore stability problems, such as shale collapse, collapse, creep and shrinkage.
4) Drilling in deep, high-pressure and low-permeability formations is facing a worldwide problem of formation pressure prediction and reservoir protection.
5) Weathered crust is developed, and drilling in carbonate reservoirs is prone to cracks and cave leakage.
6) H2S and CO2 exist in some areas, which has problems of drilling safety and preventing high-pressure gas channeling.
(2) Northeast Sichuan
The total thickness of north sea facies, land-sea interaction and continental sediments in eastern Sichuan is more than 10 thousand meters. The strata in this area are mainly characterized by high and steep structures, and the strata are severely broken, forming a number of high-strength fold zones. Deep carbonate fractured gas reservoirs generally have the characteristics of multi-layer, multi-pressure system, high pressure and high sulfur content. The Jurassic strata in the upper part of eastern Sichuan are mainly mudstone and shale with thin sandstone. Mudstone and shale have high argillaceous content, and the lower limestone has high formation pressure and is rich in corrosive gases. During drilling, complex situations such as blowout, sticking, lost circulation, collapse, well deviation, hardness and H2S are relatively concentrated, mainly as follows.
1) technical problems of deviation prevention and fast drilling in high-steep structure and large dip stratum. 60% ~ 80% of the structures in East Sichuan belong to high-steep structures with many underground faults. The dip angle of strata is generally 30 ~ 65 (maximum 85) and the natural slope is strong (the natural slope is greater than1100 m).
2) The formation has hard lithology, strong abrasiveness and low ROP. The drillability grade of formation rocks in this area is generally high, in which about 80% of the formation drillability grade of roller bit is above grade 5, and about 80% of the formation drillability grade of PDC bit is above grade 4, and the ROP is low.
3) The continental strata are easy to collapse, and there are many lost layers in the whole well, which affects the drilling speed. The upper strata in eastern Sichuan are mostly sandstone and mudstone mixed with thin shale, which is prone to hydration expansion during drilling, and the upper strata are generally inclined at a large angle, so the borehole is prone to mechanical instability. Another feature is the development of faults, cracks and caves, frequent well leakage, more malignant leakage, long treatment period and great economic losses. According to statistics, more than 80% of marine oil and gas wells in northeast Sichuan have lost circulation during drilling, of which 20% ~ 30% are serious and 10% ~ 20% are malignant. The whole well of Jinji/Kloc-0 lost 75000m3 of drilling fluid, and it took more than 6 months to deal with the lost circulation accident.
4) The prediction accuracy of formation pressure is low and the pressure system is complex. There is a multi-pressure system in the large section of eastern Sichuan, so it is usually necessary to increase the drilling fluid density to suppress well collapse. The density is generally above 1.40g/cm3, with a maximum of 2.40g/cm3. This increases the risk of leakage, leading to narrow drilling safety window and leakage in the same layer. At present, the prediction of formation pressure of marine carbonate rocks is still a worldwide problem, and there is no effective quantitative description theory and method, which leads to the lack of pertinence in oil and gas well design (wellbore structure, drilling fluid density, etc.). ) and high drilling risk. The designed drilling fluid density of Maokai 1 well is 1.35g/cm3, which actually reaches 2.2g/cm3；. . The design drilling fluid density of Heba 1 well is 1.65g/cm3, and the actual drilling fluid density is as high as 2.35g/cm3.
5) Deep burial, high temperature and high pressure, rich in H2S and CO2. The buried depth of the target layer in northeast Sichuan is generally 5000~6500m m. Because gypsum rocks are generally developed near marine hydrocarbon sources and reservoirs, thermochemical reduction of sulfate occurs at high temperature, which is rich in acid gases such as H2S and CO2, and the two gases often coexist. The average content of H2S in Puguang gas field is 14.96%, and the partial pressure is 6.89 ~ 10.46 MPa. The CO2 content is 8.2%, and the partial pressure is 4.36 ~ 5. 1 MPa. Hydrogen embrittlement occurred in Maoba 4 well in a short time, which damaged the drill string and led to the scrapping of drilling tools in the whole well. In 2003, "12.23" blowout accident occurred in Luojiajing, Sichuan, which shocked the whole country. The leakage of natural gas containing H2S has caused great damage to the surrounding life, property and environment.
(3) Yunnan, Guizhou and Guangxi
Marine strata are widely distributed in Yunnan, Guizhou and Guangxi, and deep wells are mainly concentrated in Chishui, Ceheng and Chuxiong basins in Yunnan. The depth of wells is mostly 4000 ~ 6000 m, but drilling is few and exploration degree is low. Its main geological features are:
1) There are several sets of gas-bearing strata from shallow to deep. According to the natural gas exploration practice in Chishui area, natural gas was found in 15 interval. The main gas-producing layers are Triassic Jialingjiang Formation, Triassic Leikoupo Formation and Feixianguan Formation, and upper Permian (Leping System) and lower Permian (Yangxin System) also produce gas to varying degrees. According to the actual drilling and surface display, the main oil and gas producing layers in Yangba sag are Maokou Formation of Lower Permian, Luofu Formation of Upper Carboniferous and Middle Devonian. The reservoirs in Chuxiong Basin are Triassic Shezi Formation, Ganhaizi Formation and Pujiacun Formation.
2) Diversification of oil and gas reservoir types. At present, the types of oil and gas reservoirs found in these three areas are pore type, fracture type and fracture-pore type, and the trap types are anticline, broken nose, high latency, anticline+fault shielding, sandstone lens and so on.
3) The formation pore pressure in each exploration area is different, and there are abnormal high pressure and abnormal low pressure in some areas. Chuxiong basin belongs to abnormal low pressure zone in the range of well depth 1000 ~ 1400 meters; Then it belongs to the normal pressurization area, 1.00- 1.60. Chishui sag is located at the well depth of 1800 ~ 2300m, that is, the pressure coefficient at the top of the fourth member from Xujiahe Formation to Jialingjiang Formation is 1.00 ~ 1.50, which belongs to the supercharged transition section. The future belongs to the abnormal fluctuation section, and the pressure coefficient is generally around1./kloc-0 ~ 2.10, but the pressure coefficient of Yangxin series in Guandu structure and Wang Nan structure is as high as 2.85.
Difficulties in drilling construction: ① The problems of leakage, blowout, collapse, sticking and hardness encountered in drilling in Chishui block are very prominent. According to the incomplete statistics of well 15, there are 46 lost circulation, 9 blowout 15 sticking. ② The difficulties in drilling construction in Chuxiong Basin are mainly complicated conditions such as hard stratum, lost circulation, well collapse and well deviation. ③ The difficulties in drilling construction in Nanpanjiang Basin are mainly the fracturing and collapse of complex strata such as Permian Qixia Formation, Carboniferous Formation and Datang Formation.
The marine geological and geographical conditions in Yunnan, Guizhou and Guangxi are very complicated, with low degree of oil and gas exploration, great drilling difficulty, large single well investment and long well construction period. The main technical difficulties in drilling are well control technology for deep wells and high-pressure wells, malignant leakage plugging technology for large fractures and caves, high-angle drilling technology for marine hard strata, drilling technology for extended reach directional wells and horizontal wells, and drilling technology for high-density drilling fluid to improve drilling speed.