Integrated Well Test Strategy in Unconventional Tight Gas Reservoirs- Learning and Experiences from an Actual Field Project
Authors: Jin Minquan , Zhang Wenxia , Zhang Hongli 
Published: May 2016

Tight gas and shale gas resevoirs are economically viable hydrocarbon prospects that have proven to be successful in North America. Unlike conventional hydrocarbon prospects, the formation permeability in these reservoirs is extremely low. To generate commercial production generally requires stimulation through extensive hydraulic fracturing. The mature and well-defined methods of well testing programs and data analysis techniques, although applicable, are often impractical because of excessive test durations. The absence of an effective well test program for these unconventional plays makes the prediction of economic productivity and fracturing success risky.

This paper presents an integrated well test program that was developed and implemented for reservoir characterization and formation evaluation of a tight gas reservoir in the Sichuan basin in southwest China. The program makes use of a combination of various formation evaluation techniques including diagnostic fracture injection test, perforation inflow test, pressure buildup test, rate transient analysis, production logging, etc. The integrated well test program was designed and modified from the conventional methods to meet the project delivery timeline and cost constraints, while responding to the challenge of properly testing the tight gas reservoir. The various tests described here were implemented during operations from well completion to trial production periods and used to estimate essential reservoir and hydraulic fracture parameters for assessment of well expected ultimate recovery (EUR). Understanding individual well EUR is the main factor deciding project feasibility.

This paper presents well test and data analysis results and summarizes the challenges encountered and lessons learned from field operations. Specifically, we present field operation results and explain what program elements have, or have not, been feasible in the unconventional gas environment. The lessons learned and experiences gained from the field operation presented here provide valuable guidance for future tight gas exploration and development operations in general.

Introduction

The northwestern & central areas of the Sichuan Basin have been identified as having basin centered gas (BCG) potential. In particular, there are several blocky sand intervals in the late Triassic Xujiahe formation that contain a relatively immature, continuous gas accumulation. The tight gas project described in this paper is located in the central uplift area of this basin and is operated by Shell and PetroChina as a joint venture. In this tight gas block, the Xujiahe formation has a gross thickness of greater than 500 m and is found at 3,000–4,000 m drill depth. The hydrocarbon (HC) system for the Xujiahe formation in the block is an overpressured section of tight gas sands that are sourced by imbedded and surrounding shales and coals. Gas and condensate can be produced from this objective section through massive multistage hydraulic fracturing treatments (see Figure 1).