Resistivity, Imaging-While-Drilling Tool Helps Well Placement in Chinese Tight Reservoirs

Abstract

Technology Update Two case studies describe the use of a new resistivity and imaging-while-drilling tool that was used to improve well placement in the Sichuan Basin of central China—a drilling environment presenting high levels of geological uncertainty. The first case involved the technology’s application in a very thin tight dolomite gas reservoir, and the second case involved the first commercial development well in the region to target a carbonate oil reservoir. In addition to assisting well placement, the technology enabled valuable information about the natural fracture network to be obtained to support the completions design. Background The Sichuan Basin holds the biggest gas reserves in China and is the country’s leading gas production region. The basin rests in a major compressional tectonic area, highly compressed and characterized by thrust-faulted anticlines. Modern exploration and production started in the 1950s. Most of the gas and modest quantities of oil are produced from fractured carbonate and low-permeability sand-stone reservoirs. PetroChina Southwest Oil & Gas Company (SWOGC)—one of the biggest subsidiaries of PetroChina—operates in the basin. It has more than 100 exploration and development concessions in the region, covering a total of 180 000 km2. Natural gas reserves in place are 842.2 billion m3, with estimated recoverable reserves of 320 billion m3. During the early phases of exploration and development, wells were drilled vertically. Horizontal wells were introduced during the 1990s to enhance production from tight reservoir plays, which include limestone, dolomite, and tight sands. Logging-while-drilling (LWD) applications used for formation evaluation and well placement typically included propagation resistivity, density/porosity measurements, and real-time density image logging. Today, after more than 20 years of production, most of the thickest tight gas reservoirs have been intensively developed. Since 2011, PetroChina SWOGC has increasingly targeted ever thinner tight gas formations using long horizontal drilling combined with multistage hydraulic fracturing. However, several subsurface challenges needed to be overcome for these efforts to succeed, particularly because of the extensive faults and fractures in the reservoir rock. To meet these challenges, and enhance reservoir understanding, the company has deployed advanced LWD real-time acquisition and transmission technology providing high-resolution electrical borehole images, azimuthal gamma ray, and multidepth formation resistivity measurements.