Engineering Geology Features And Drilling Strategy In Heba Structure, Northeast Sichuan

The geological conditions of Heba structure are complex, and so far by now the main target formations in the current exploration and exploitastion are the T1j2 and T1f3 with marine facies. Due to the marine facies carbonate reservoir is deep buried, the depth of target strata in T1f3 could reach more than 5000m, the formation temperature and pressure could be high, the gypsum rocks could develop well and hydrogen sulfide could be found in T1j2. As the dip angle of terrestrial clastic rock formation is steep, the rock is tight and the sandstone and mudstone interlayered frequently, the drilling capacity is poor and the wellbore is unstable. The existing of Vertical multi-pressure system and extremely complex of engineering geology features lead the accidents occur easily while drilling such as leaking, collapsing, well kicking, jamming and well plugging. The low drilling rate and long term drilling lead the drilling and completion projects confronting server challenges and would have great influences on the schedule and consequently restrict the evaluation of gas reservoir exploration and exploitation in study area. The previously domestic and international studies show that the origins of complex problems in wells are closely related to the stress. This dissertation is focusing on varies of complex conditions confronted in drilling wells in Heba structures and is starting analysis from the basic rock physical parameter. It is guided by theories and methodologies of petroleum geology, structural geology, geophysical well logging, reservoir geology, rock mechanics, drilling engineering, oil and gas reservoir reconstruction, reservoir engineering and etc., combined with rock mechanics parameters and stress measurement experiments and based on the deliberate understanding of the study area. In the dissertation, the actual drilling mechanics parameters of rock formation, stress, pore pressure, collapse pressure, fracture pressure and other geological parameters logging prediction models and their corresponding profile have been established, the borehole stability analysis has been carried out and the drilling strategy corresponding to complex engineering geology situations has been proposed, and it provides a scientific basis for designs and implementations of drilling and completion programs in Heba structure.Through the research, the following understandings have been achieved: (1) the overall performance of the rock in Heba is tight, strong, hard, and poor in drilling; (2) pore pressure vertically apparents obvious banding phenomenon in Heba structure and pore pressure in the marine facie carbonate rock is abnormally high; (3) the minimum and maximum horizontal principal stress gradients of the terrestrial clastic rock are 2.30-2.70MPa/100m and 2.7-3.1MPa/100m, the minimum and maximum horizontal principal stress gradients of marine facies carbonate rocks are 2.13-2.50MPa/100m and 2.65-2 .90MPa/100m, and the maximum horizontal principal stress direction is nearly east-west between 70°-110°due to tectonic movements of marine facies carbonate rocks; (4) the collapse pressure gradient and fracture pressure gradient of terrestrial clastic rock is relatively high and the density window of safety drilling fluid in terrestrial clastic rock is relatively wide while in the marine facies rock the density window is comparatively narrow, stress collapse pressure gradient in T1f and T1j is between 0.3-0.48MPa/100m, the collapse pressure is minimum, fracturing is maximum, the well bore is most stable when directional wells drilling towards south and north, the density with 1.59g/cm3 in directional drilling wells and vertical wells could maintain the mechanical instability less than 90°in Heba structure; (5) the leakage occurs in Heba structure is mainly due to the well developed fractures and the drilling fluid would flow to low pore pressure area as pressure differentiation, the main reason for well collapse and pipe stuck is that the continental facies rock in well would absorb water and expand and consequently exacerbated wall collapse and resulting stuck under the condition of formation water or drilling fluid filtration; (6) through summarizing the complexities of engineering geological characteristics, the drilling method, casing, drilling fluid density and other engineering process optimization has been proposed, the suggestion is that the wellbore configuration should be like this: seal shallow water layer with thickness of 150m during spud, seal the water layer at the J2s during second spud, seal the low pressure drain layer in T3x, T2l and other formations during third spud, drilling the high pressure T1j and T1f and finish drilling in target zone, This approach could not only balance pressure in each spud and reduce leakage, but also create opportunities to prolong the gas drilling procedure as long as possible in second and third spud, besides, the density window of safety drilling fluid is also suggested as 1.15-2.37g/cm3 in spud , 1.47-2.13g/cm3 in second spud, 1.76-2.12g/cm3 in third spud, 2.08-2.26g/cm3 in forth spud.The research findings of this dissertation are strongly targeted and highly tallied with the actual drilling practice, and the forecast results is highly reliability. The research results provide a firmly technical support to strengthen the application of drilling and completion technology, to ensure the scientific, efficient and safe drilling procedures and to implement the measures such as fracturing and alternation in the study area. A systematic research methodology of engineering geology features covering the experimental test, rock mechanics parameters explanation, the stress explanation, the three pressures explanation, comprehensive analysis of wellbore stability and the corresponding drilling strategy has been established and proposed, and it is a good reference for the same type of gas pools home and abroad.