3D Reservoir Geomechanics Workflow and Its Application to a Tight Gas Reservoir in Western China
Authors: Qiu Kaibin , Cheng Ning , Ke Xiangui , Liu Yang , Wang Lirong , Chen Yingru , Wang Yong , Xiong Pi 
Published: May 2016

Easy oil has gone and now the focus of exploration and development in China has shifted to tight reservoirs deemed techanically challenging. One of the key challenges in tight reservoirs is how to place and land horizontal wells in sweet spots (with high reservoir quality and completion quality) and how to stage-fracture these wells efficiently to produce these tight reservoirs economically.

The paper presents a new 3D reservoir geomechanics workflow that has been applied to a tight gas reservoir in western China. The reservoir is very deep (up to 4500m) and the production rates from the wells are very low. Some hydraulic fracturing had been conducted for vertical exploration wells but the post-fracturing production rates were still not satisfactory. The best chance to produce this tight reservoir is to place horizontal wells in the areas with the best reservoir quality and completion quality and carry out optimized multistage hydraulic fracturing. To this end, a 3D full field geomechanics model was constructed through integration of seismic data, geological structure, core data and log data. This 3D geomechanics model enables a 3D identification of the high completion quality (high fracturability) zones in the reservoir and subseqently placement of a new horizontal well. A 1D mechanical model was then extracted along the planned trajectory from the 3D geomechanics model. Based on the 1D geomechanics model, optimization of the stage-fracturing design was conducted to obtain the optimal number of stages, optimum fracture half length and optimum staging.

Introduction

Multistage fracturing of horizontal wells has been successful in gas shale (Chong et al. 2010; King 2010) in North America and brings promise to development of many tight reservoirs around the world. However, experience shows that in many areas this technique has failed to achieve initial production rate goal (Ketter et al., 2006; Britt and Smith, 2009). One of the root causes is that heterogeneity of the reservoir was largely ignored, which led to poor placement of the horizontal wells and/or bypass of large portion of reservoir because of poor initialization and propagation of hydraulic fractures in multiple stages. These results highlighted the importance of 3D geomechanics modeling, to quantify the heterogeneity of the reservoirs and to enable the optimum well placement and hydraulic fracturing programs.

To this end, a new 3D reservoir geomechanics workflow was developed and applied to a tight field, named XG, near Karamay, Xinjiang, in western China. The target reservoir formation is the Jamuhe Permian tight sandstone with porosity of 8% and relative permeability of about 0.01mD. The preliminary exploration indicated a large gas reserve in the reservoir. For such tight reservoir, the economic success for gas production depends entirely on the effective generation by hydraulic fracturing of adequate surface area to flow (Economides and Nolte 2000). Since the reservoir is deep and very tight, the drilling and completion costs will be high, and a production rate up to 100,000 m3/d is required for economics. However, previous stimulation of vertical wells in this field did not yield satisfactory results, achieving a peak production rate of only up to 27,000 m3/d. To optimize the planning of horizontal well multistage fracturing, understanding the heterogeneity of the reservoir is the key. Through the 3D reservoir geomechanics workflow, a bettter understanding of heterogeneity of reservoir was obtained and optimization of hydraulic fracturing was able to be conducted.