Mineralogy, Fracture and Textural Analysis for Formation Evaluation in Tight Gas Reservoirs

摘要

Tight gas reservoirs are generally defined as having permeability less than 0.1mD and requiring hydraulic stimulation to produce commercially. Their low permeability is a consequence of a complex diagenetic history where compaction and mineral growth reduces the pore and pore throat sizes. Tight gas sands are also often deeply buried and subject to large stresses. The formation evaluation process of these reservoirs therefore consists of 5 components: lithology (mineralogy), texture, depositional environment, present day stress, and tectonic history (fracture types and orientation). The successful integration of these geological, petrophysical, and geo-mechanical disciplines is necessary in order to evaluate these reservoirs. Because of their mineral and textural heterogeneity and their low porosity, the fluids and reservoir properties of tight gas reservoirs are more difficult to evaluate than those of conventional reservoirs. The objectives of early formation evaluation -performed before well completion - in tight gas reservoirs are also different; rather than aiming for a static reservoir evaluation (porosity, saturation) the priority goes to locating the zones most favourable for hydraulic fracture stimulation. Operators often express their strategy for tight gas reservoirs evaluation as: [1] confirm hydrocarbons location, [2] confirm hydrocarbons mobility, and [3] perform reservoir characterization (porosity, saturation) (Ref. 1). Others express their concern as: "what is the strategy for hydraulic fracturing when there is no sign of gas?" Clearly, the evaluation of hydrocarbon volumes and reservoir productivity in tight gas reservoirs requires targeted measurements and evaluation methods to locate and quantify hydrocarbons, and assess the reservoir productivity before and after hydraulic fracture stimulation. We present here a few concepts and methods to complete a mineralogy, fracture and textural analysis of these reservoirs. (Although they have clear applications to the evaluation of tight gas sands, we do not describe methods for geomechanics, or formation testing and sampling. Neither do we expand on the evaluation uncertainties brought by low porosity and possible bad borehole conditions.)