Guang'an gas field in Sichuan Province, whose discovery was officially announced by CNPC in August 2005, is a large-scale gas reservoir that has in excess of 100 billion m3 of natural gas reserves. The main sandstone reservoir in the Guang'an field, Xujiahe formation, mainly consists of Xu-2, Xu-4, and Xu-6 formation. The lithology of the Xu-6 formation, which is the main reservoir section, where the lithology mainly consists of fine-medium feldspar-quartzite sand, lithic feldspar-quartzite sand, the pore type dominated by inter-granular, small inter-granular and intra-granular dissolved pore. The porosity generally ranges from 4–10%, with permeability ranges of 0.241–1.116 mD, showing a medium porosity and low permeability characteristics.
Small reservoir pore throat structure and low permeability nature of the rocks makes normal gas production with conventional perforations very restrictive, and will require proppant fracturing stimulation treatments to produce the gas economically. The conventional method to complete the wells in the Xujiahe formation is to drill vertical or slightly slanted wellbore, cased with 5.5" casing, perforate and carry out proppant fracturing treatments. Fracturing treatments in vertical wells yield mixed results, mainly due to strong heterogeneity in the lateral section of the reservoirs.
The concept of drilling horizontal wells has been developed in the Guang'an field, with the intention to tackle the large resource potential within the wide sand distribution, and highly heterogeneous gas-bearing formation. Implementing the horizontal wells concept at the same time confronted with the challenge that involves drilling, completion and stimulation measures.
This paper describes a case history of how a multi-stage technique was used to stimulate a poor-performing horizontal well that was completed open-hole. The technique used a selective openhole completion tool that can allow the wellbore mechanical isolation into four intervals based on reservoir evaluation. Each isolated interval then is individually proppant fracture stimulated in a single operation, and once completed, gas is produced from all intervals in a co-mingled manner. The treatment utilized 200,000 lbm of 20/40-mesh intermediate strength proppant carried with 3,700 bbls of delayed borate crosslinked fracturing fluid. The whole pumping treatment took less than five hours to complete. The tested stabilized measurement indicates natural production of gas at 5.8 mmscf/day, which is more than 20-times as compared to the pre-frac gas rate.
The success of this project, which is the very first in East Asia, is considered to be a major breakthrough in China, for PetroChina in particular, because the technology has proven to be an effective stimulation method to increase well productivity from horizontal wells that are completed open-hole. This opens up a window of hope for the dilemma of many horizontal wells that have already been drilled but perform poorly.
