Challenges of Horizontal Well and Successful Cases for Tight Gas Development in China
Authors: Ailin Jia , Dongbo He , Chengye Jia , Guang Ji , Yunsheng Wei 
Published: May 2016

Abstract
Low production rate and low estimate ultimate recovery are the most negative factors for gas well in tight gas development. Horizontal wells which can effectively improve the flow matrix and increase the drainage areas are the common practices to improve production level and EUR. Compared with conventional gas reservoir, horizontal drilling in tight gas reservoir are more complicated as "sweat spots" are distributed more unevenly. Challenges of horizontal drilling still remain in effective well track to penetrate gas bearing sands as much as possible.
Technique innovations have been developed from three aspects for horizontal well in Sulige tight gas development. Firstly, well location selection should be conducted under the guidance of detailed geological researches. Scale and distribution of sands should be investigated thoroughly. Sand geometry and superimposed mode are key factors for geometric arrangement of horizontal drilling. Comprehensive data including seismic, logging and core are of great value in sand distribution prediction. As strong heterogeneity in fluvial sedimentary system, these researches can ensure high quality in well location optimization and well track design. Secondly, measurements while drilling (MWD) are the effective technique support to enhance drilling success rate. Surveillances while drilling including gamma log and gas logging are selected for well path control. Gamma log has good response to lithology and gas logging has its priority in diagnose of gas bearing sands. Through analysis to drilling data, muddy intercalation and microfault can be identified and well track can be adjusted promptly. Thirdly, stimulation treatment such as multi-stage hydraulic fracturing is the most effective way for gas well to increase its deliverability. Compared with traditional fracturing, it accommodates multiple zone stimulations in a single operation and flow matrix can be effectively improved. With the progress in fracturing, well deliverability and economic benefits have been significantly increased.

Introduction
Tight sand gas as one of main unconventional gas reservoirs, display the most rapid development in recent years throughout the world. Since its successful development in U.S.A, this type of resource now plays an increasingly important role in world natural gas resource market. China occupies a large amount of tight sand gas resource and has been obtained a good performance in exploration and production of this resource during the past National 11th Five-year Plan period. Sulige gas field in Ordos Basin[1], Xujiahe gas field in Sichuan Basin[2–5] and Denglouku gas fields in Songliao Basin[6] show typically successful exploration and production of tight sand gas in China. Compared with geological settings and development backgrounds in North American, tight sand gas in China has characteristics including poorer original gas in place (OGIP), deeper buried depth and lower net pay thickness. Taking Ozona gas field and Sulige gas field as examples, permeability, porosity, target depth and net pay thickness are of great differences. Canyon sandstones in Ozona have mean permeability of 0.27mD, porosity between 9~15% and net pay thickness ranged between 6.1 and 30.5m. Low Permian Shihezi sandstones in Sulige have mean permeability of 0.98mD, porosity of 9.1% and net pay thickness of 8m[7]. Lower porosity and net pay thickness are reasons which result in poor richness for OGIP, low EUR/well and poor benefits.