Geomechanical Characterization of Longmaxi Shales and its Relevance to Hydraulic Stimulation Performance in the Southern Sichuan Basin

Roger Yuan1, Fa Dwan1, Ian Zhang1, Liang Jin1, Navpreet Singh1, S. Wang2, X. Li2, and M. Lin2
1Shell China Exploration and Production Company; Chengdu, Sichuan, China
2PetroChina SouthWest, China

Shale gas reservoirs in compressional tectonic settings are commonly composed of shale matrix with uneconomically low porosity, permeability, and potentially high retention risk. To effectively and economically recover hydrocarbon from such tight reservoirs, it is important to optimize well placement and hydraulic fracture stimulation in order to maximize the stimulated rock volume. To achieve this, the application of geomechanics plays a critical role in determining subsurface stress field, identifying brittle and fraccable shale intervals and predicting natural fracture network in a structurally complex setting.

The lower Silurian Longmaxi shale distributed in the Southern Sichuan basin is characterized by a lower section of graptolite-bearing black to dark-grey, calcareous-siliceous shale, generally several meters thick, overlain by an upper section of grey calcareous shales/mudstones and siltstones, interbedded with an increasing number of thin limestone layers towards the top of the formation. The averaged total porosity of the shale package is generally low, and the averaged permeability is typical of other shale gas plays in the basin. Subdivision of shale reservoirs is based on the presence of markers, log motifs and petrophysical characteristics, and is broadly divided into Zone 4 to Zone 1 in descending order. There is a general decrease in silica components from Zone 1 to Zones 4. The link between high quartz and TOC content with favorable porosity and permeability makes Zone 1 the best reservoir quality in Longmaxi shale within the intervals of interest.

To produce from such ultra-low matrix permeability shale formations, hydraulic fracture stimulation is demanded to increase in-situ permeability and to enhance accessibility to reservoir hydrocarbons. In this sense, geomechanics becomes an important element as it assists in assessing shale fraccability which includes subsurface stress states, mechanical properties, rock fabrics and expected stimulation patterns.

The purpose of this paper is to give an overview of the preliminary evaluation of stress setting, geomechanical properties, stimulation potential and related technical challenges of the Longmaxi shale play in the Southern Sichuan basin. By means of reservoir characterization and modeling undertaken at different scales, geomechanics provides an opportunity to unlock the shale play towards commercial development.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013