Employing Both Damage Control and Stimulation: A Way to Successful Development for Tight Gas Sandstone Reservoirs

Abstract Tight gas sands exhibit extremely low porosity and permeability, so most people think that it is impossible to produce gas economically without stimulation. During past several decades, great efforts have been paid to improve hydraulic fracturing, but the results are mixed. Formation damages induced by drilling and completion fluids to natural fractures and tight matrix retard artificial fractures to give full conductivity. Tight gas sands in Western Sichuan Basin are characterized with natural fractures developed in heterogeneity, complexity in micro-macro multi-scale structures, and very sensitive to environmental changes. Drilling fluids invasion into tight formation mostly occurred through natural fractures, which leads to solid plugging, water trapping, clay mineral impairments, chemical adsorption and retention. The paper presents application cases of temporary plugging fluids for naturally fractured sands in two tight gas fields. As the invasion depth localized to within 3cm of the well-bore, perforation can bypass the cake easily and efficiently. Wells with damage control give relatively higher initial production and the production rates after hydraulic fracturing treatments are highest owing to the fact that artificial fractures can connect natural fracture systems and matrix without damages. These wells also show stable and high productivity with longer periods and output satisfactorily economical results. Introduction The natural gas from tight sands, being with a huge amount of resource, occurs in many petroliferous basins around the world. During the past decades, most of the exploration and production activities of this unconventional resource have been concentrated within the northern American. For example, the total production of the tight gas reached to 705.3×108m3 in 1994. Nowadays, this kind of resource has been paid much attention in Europe and Asia Pacific areas and tight gas production increases rapidly though the total amount is relatively at low level. The tight gas resource in China is estimated as much as 12×1012m3, which mostly exists in Sichuan, Ordos, Songliao, Tarim, Zhunger, and Chidam basin. There is a obvious contrast between its huge amount of tight gas resource and production or reserves owing to the fact that the damages during well drilling and completion generally happened, sometimes leaded to serious impairments. The situation heavily retards the improvement of exploration and production. The America and Canada are the pioneers for the successful development of tight gas. At the early and middle stages of tight gas development, the damage control during drilling and completion has not been paid as much attention as that during stimulation. It is widely accepted that the natural productivity is too low to be economically exploited. And it is impossible to successfully produce gas from this type of reservoirs without stimulation. Therefore effective stimulation is regarded as the key technology for tight gas development. Under the direction of this opinion, a great deal of efforts has focused on the improvements of stimulation. But the application results of stimulation for tight gas formation are mixed, and usually disappointed. During the middle of 1960's, the projects for tight gas supported by DOE of US. Concentrated on the nuclear explosion stimulation. Then it turned to massive hydraulic fracturing (MHF) for some reasons. But most of MHF has not achieved to the prospected production including the multiwell tests) (MWX. An explanation about the facts is that the stimulation scale or fracture length is not large enough to connect more volume of reservoirs. Following this idea, stimulation scale increased continuously, and super massive hydraulic fracturing appeared. Application of these techniques show that enlargement of stimulation scale will get half the result with twice the effort unless the damage to natural fracture systems from aqueous based fluids has been prevented1.