YM2 Oilfield is an ultra-deep carbonate oilfield located in the northern uplift of Tarim Basin in western China. We have identified 108 seismically resolvable faults formed over three periods. These faults control the distribution of oil and gas accumulation units. Permeability differs significantly amongst various faults, and various sections of the fault planes. There has been no effective quantitative evaluation measure on the relative opening or sealing of faults for a long time, the sealing capacity of the faults have not been evaluated so far. This has a negative impact on the evaluation of accurate reservoir compartmentalization and partitioning, and evaluation of various oil and gas flow units which are critical for constraining the development program and production performance of oil field.
There are many geological factors that influence the permeability of faults, such as burial depth, fault throw, dip angle, strike, lithology variation, pore pressure and the in-situ stress field. After evaluating these factors in the YM2 oilfield, we determined that the critical factor that controls fault permeability is the geomechanical response of faults under the current stress field. In order to determine if the faults are permeable relatively in the current stress state, we established the geomechanical model of YM2 reservoir which describes the present-day stress regime, along with the vertical and horizontal distribution of the geomechanical parameters of this reservoir. Based on the interpretation of 3D seismic data, we characterized the spatial combination relations of faults in the reservoir and extracted the occurrence information of each fault according to certain step size. Then, we calculated the normal and shear stress acting on the various fault planes in order to evaluate whether those faults are permeable relatively in the current stress state.
It is shown that the fault zone not only controls the evolution of local structure, but also significantly impacts the regional stress field, which in turn impacts the geomechanical response of fault zone and their permeability. The NW strike-slip faults in the southeastern part of the oilfield in particular are characterized by high values of normal stress and low shear-to-normal stress ratios with relative lower permeability. Producing wells in this region have low productivity and weak connectivity amongst wells. In contrast, the northwestern area that has NE strike-slip faults and NS thrust faults have low stress but high shear to normal stress ratios, and hence relatively higher permeability. The surrounding wells have higher inter-well connectivity and are more productive. In the same fault zone, as the relationship between stress and fault orientation changes, the potential mechanical behavior also affects permeability variation and well productivity. The main reason is that the interaction between faults and stress field leads to the increased reservoir heterogeneity in the fault zone or among faults. Based on this concept, we chose several advantageous well locations, where horizontal minimum principal stress is low, anisotropy of horizontal stress and shear-to-normal stress ratio are high.
This study classified the faults of YM2 oilfield based on geomechanical response, clarified permeability variation of various fault zones and their impact on productivity, and then it provided the quantitative selection basis for well placement and wellbore trajectory optimization.