Geomechanical Dilation Assisted VHSD Process in Altered-Stress Mature Oilsands Reservoir: Geomechanical Studies and Field Experiences
Authors: Chen S. , Xiang H. , Li B. , Xu B. 
Published: September 2022

ABSTRACT:
Petrochina’s Karamay oilsands reservoir features high in-situ oil viscosity and strong reservoir heterogeneities. Early depletion plan envisioned a cyclic steam stimulation (CSS) process through vertical wells. As CSS efficiency declines due to lack of steam confinement and slow steam chamber growth, vertical-horizonal well steam drive (VHSD) process becomes an attractive recovery scheme in this mature oilsands field. We chose to use geomechanical dilation to create the dilation zone that connects the vertical wells and infill horizontal well. As the magnitudes and orientations of in-situ principal stresses affect the propagating direction of dilation zone, the geomechanical study of dilation process in altered-stress mature oilsands reservoir becomes a key design factor that controls the efficiency of inter-well communications. Based on laboratory and field characterization results, we conducted coupled geomechanical and thermal reservoir simulations to establish the in-situ stress field that altered by 20 years of CSS process and simulate the dilation zone propagation in the altered stress reservoir and inter-well communications. Our numerical simulation results indicate that, to increase the possibility of inter-well communication, dilation zone needs to be initiated on the newly drilled infill horizontal wells.

1. INTRODUCTION
The H98 oilsands development block at Petrochina’s Karamay oil field is Jurassic deposit, and the major oilsands producing zone is G1 layer. Sedimentary facies of the reservoir primarily consist of braided fluvial and branched fluvial facies. Deposits in this region are influenced by the unsteady flow of water and thus show a complex sediment composition and a wide range of particle sizes. The reservoir thickness ranges from 12.0 to 20.0 m, with an average reservoir thickness of 14.0 m. The reservoir sands are medium-fine sand; the loose cementation with mud content is approximately 8%; reservoir porosity is in the range of 27%-32%; oil saturation is 65%-75%, and reservoir permeability is approximately 1000-1500 md. Bitumen has high viscosity under reservoir conditions (for example, oil viscosity is more than 1 000,000mPa·s under reservoir conditions and changes to 20,000 mPa·s at 50 °C). The development of H98 oilsands block started since 30 years ago and early depletion plan envisioned a cyclic steam stimulation (CSS) process through vertical wells. The CSS process involves injecting steam into the reservoir above the fracture pressure, allowing the reservoir to undergo a short period of "soaking", and then flowing back and/or pumping the same well until the produced fluid declines to such a point that it is time to initiate another steam cycle (Bulter, 1997). Currently, CSS is still a primary thermal recovery method used in H98 block. After three decades of successful CSS operation, some of the CSS wells have evolved into a mature (late-life) stage with a trend of declining thermal efficiency, including fast declining of oil rates and very high water saturation. For example, the averaged CSS cycles at H98 block is 8.3 cycles and the averaged oil rate from single vertical wells is 1.4 T/day with more than 90% of water content. Fig. 1 gives the averaged water saturation on H98 block from year 2000 to 2018. To enhance the late-life CSS process, several alternative thermal recovery processes have been deployed and pilot tested in the field. One of these alternative thermal recovery processes is vertical-horizontal well steam drive process (VHSD).