Study on Well Pattern Optimization of Tight Sandstone Gas Reservoir with the Strong Heterogeneity
Authors: Zhang Ji , Lu Tao , Li Yuegang , Yu Shuming , Li Jingbu , Hao Qian , Li Fuping , Xu Wen , Xue Wen , Danfu Wan 
Published: March 2015

Introduction
Sulige gas field, located in Ordos basin, is a representative and the largest unitization tight sand gas field in China, reserve quality more than 30000×108m3, the largest Gas-Initially-in-Place in China. The average reservoir thickness is about 9m, the porosity is 9%, and the permeability is 0.73×10-3µM2. After 8 years development, it is expected that the gas annual production will be over 200×108m3 by the end of 2013.
The main types of He8-Shan1 reservoir of Permian system of in Sulige gas field are braided river sand bodies and meandering river sand bodies, which overlay vertically and composite laterally in the various period. The well pattern will directly affect the gas field recovery and economic benefits. The preliminary evaluation of Sulige gas field shows that, if recovery ratio increase 1%, gas production may increase by tens of billions cubic meter. It is obvious that gas recovery ratio is one of the keys to enhance the level of gas field development and benefit, and optimizing development well pattern is the most important element to improve the gas field recovery ratio. Changqing Oilfield Company has established three trial blocks since 2003 (Su6 encryption area, Su14 encryption area, Su14 three-dimensional test area). Around effective sand body anatomy and interference test, we have done a lot of researches and tests. In the paper, a lot of methods and technologies are applied to optimize well pattern in Sulige gas field, such as the geological knowledge database statistics, outcrop observation, well testing, fine description in the dense well space test areas, well interference testing, geological modeling and numerical modeling.

Overview of Sulige gas field
Sulige gas field is located in the Inner Mongolia Autonomous Region in China. Regional tectonic belongs to the northern belt in northern slope of Ordos Basin. The exploration area is about 40000km2, total resource quantity is about 3.8×1012m3. It is the largest onshore gas field and the typical tight sandstone gas reservoir. Sulige gas field was discovered in 2001. From 2006, the gas field has been explored and developed on a large scale. By the end of 2012, the production capacity annually reaches more than 200 billion cubic meters.

The difficulty of development well optimization in Sulige gas field
The Upper Paleozoic reservoir of Sulige gas field has a typical characteristic of fluvial-delta deposits, and the sedimentary characteristics are obvious in the different disposition period. H8 formation can be divided into 4 small layers, S1 divided into 3 small layers. H8 formation's thickness reaches 60m~70m, and the average thickness of sand reaches more than 30m, sand/stratum thickness ratio closes to about 50%. There are about 10 sand bodies overlaid vertically. The multiple sands are mutually superposed laterally, with large area of distribution and high sand/stratum thickness ratio, but sand properties change rapidly, which causes the reservoir gas-bearing property laterally changes quickly and heterogeneity is very strong. The sedimentary micro facies are most mid-channel bar, point bar and channel. The phenomenon of gas-bearing sand bodies wrapped by dry sand bodies is common, the favorable sedimentary microfacies are mid-channel bar of braided river and point bar of meandering river. So the difficulty of development well optimization in Sulige gas field is below.
The difficulty of recognizing the effective sand body. Although sand distributes widely and the continuity is comparatively good, the effective sand is poorly continuous and small. In addition, the reservoir buries deeply, effective sand is thin. These characteristics caused that it is difficult to identify the effective sand by the seismic technology. So it is vague to cognize the effective sand's geometrical morphology, scale and distribution.