Infill Drilling Optimization in Waterflooded Tight-Low Permeability Reservoir
Authors: Yu Tao , Lei Zhengdong , Li Jiahong , Hou Jianfeng , An Xiaoping , Zhou Xiaoying , Deng Xili , Wang Jinfang 
Published: October 2018

Objectives/Scope: Waterflood development in low permeability sandstone reservoir is characterized by poor sweep efficiency and fast water breakthrough. Infill drilling has been developed in China for decades as a method of accelerating production and increasing ultimate recovery for such mature waterflooded field. However, optimizing infill drilling pattern entails additional challenges because of the complicated remaining oil distribution affected by reservoir heterogeneity and multi-scale fractures after long-lasting production.

Methods, Procedures, Process: The proposed workflow is a four-step methodology based on a case study of tight oil sandstone reservoirs (average permeability between 0.3mD to 10mD) in Ordos Basin, which is the second largest oil-bearing basin in China. Firstly, a statistical analysis and dynamic diagnosis using real data were applied in order to evaluate waterflood performance. Secondly, the dynamic characteristics of each category were identified by integrating decline analysis, injection/production profile, tracer monitoring and well testing interpretation etc. Moreover, the densely- spaced inspection well core data indicating remaining oil distribution and flush zone was observed. Thirdly, the dominant factors influencing production were investigated considering geological features, waterflooding injection intensity, in-situ stress field etc. Finally, different infill drilling scenarios were simulated and optimized based on the understandings and the field implementation results were presented.

Results, Observations, Conclusions: Three typical production modes and a diagnosis chart were presented indicating effective drive, watered out and poor drive, respectively. The watered out producers performs as drastic water breakthrough, sudden drop of hall plot, sharp spike of GR curve in injection profile and significant amount of tracer production, whereas, poor drive wells act like depletion mode with continuous production drop and extremely low pressure maintenance. Multi parameter analysis explains fracture propagation and reservoir heterogeneity are the dominant factors in watered out region, while oversized well spacing results in poor drive performance, which is testified by real core samples from newly drilled inspection wells. Numerical simulation results indicates that: an optimized staggered line-drive pattern gave the best result for watered out region and the predicted recovery at 95% water cut improved 6.6% of OOIP. This infill drilling optimization methodology was successfully implemented in WY Reservoir and resulted in 6% decrease in water cut and 13% increase in production rate. The estimated ultimate recovery (EUR) improved about 5% of OOIP.

Novel/Additive Information: This paper provides an optimized infill drilling methodology and a case study for better understanding the production performance and infill drilling workflow in waterflooded tight oil sandstone reservoirs. It offers a guidance for future infill drilling of similar reservoirs.