Research On Multiple Migration Mechanisms And Well Testing Theory For Shale Gas Reservoirs

Shale gas is a kind of self-sourcing and self-preserving, unconventional natural gas which is stored either by compression in natural fractures and pores or by adsorption on the surfaces of the solid material. Acceleration of shale gas development has become increasingly important to China’s energy supply and economic growth.Shale gas reservoirs have complex, multimodal pore-size distributions and unique storage properties, causing shale gas to be transported by multiple flow mechanisms through the pore structure which include desorption from the surface of kerogen, diffusion in low permeability shale matrix and laminar flow in natural fractures and big pores. Compared to conventional gas reservoirs, the development of shale gas reservoirs is much more difficult. Despite the successful development of shale gas in the US, the study over the migration mechanisms and percolation theories of shale gas is far behind the field practice. Current gas flowing theories become inadequate to describe gas flow in shale gas reservoirs and to predict production dynamics, which will inevitably have an adverse effect on the economic and efficient development of shale gas reservoirs.Based on the literature review, this dissertation carried out an in-depth experimental study over the characteristics of pore structure in shales, unique storage mechanisms, adsorption-desorption behavior and multiple flowing mechanisms of shale gas. Appropriate models are selected to describe different migration mechanisms in shales, including adsorption-desorption, diffusion and viscious flow. Based on different considerations of flowing mechanisms in shale matrix, this dissertation establishes different basic flowing models for shale gas reservoirs. The pressure responses of different well types in shale gas reservoirs are then obtained by employing point source function. Type curves are plotted with Stehfest numerical inversion algorithm and different flow regimes are identified. The thesis also presents the discussion over the effects of different parameters, related to adsorption-desorption, hydraulic fracturing, etc, on type curves and flux distribution. In the final part, some theoretical models proposed in the dissertation are verified by actual field data.The major conclusions of this dissertation can be summarized as follows:(1) The characteristics of microscopic pore structures of shales, classification of storage spaces and pore size distribution in shale gas reservoirs are illustrated based on the laboratory experiments.(2) Isothermal adsorption experiments are conducted under shale gas reservoir temperature, and proper mathematical model is established to describe adsorption and desorption phenomena in shales. The impacts of several relavant parameters on the adsorption volume of shale gas are analyzed, and a quantitative relationship between the maximum adsorption volume of shale gas and total organic content (TOC) is obtained by regression of the experimental data.(3) Shale gas flow in natural fracture system is believed to be laminar flow, and different basic coupled flowing models are established according to different considerations of transportation mechanisms in shale matrix. In the laminar flow-diffusion dual porosity model, as flow in shale matrix is assumed to be desorption and diffusion. The effect of desorption is aken into consideration by introducing the adsorption coefficient, and both pseudo-steady and transient models are presented regarding different interporosity diffusion between shale matrix and natural fracture. In the laminar flow-laminar flow coupled with diffusion dual porosity model, gas flow in shale matrix is assumed to be the combination of desorption, diffusion and laminar flow. The effect of adsorbed gas is incorporated in the total compressibility of matrix system, and the combined effect of laminar flow and diffusive flow is represented by an apparent permeability of matrix system.(4) Point source function is adopted to obtain the basic point-source solution for each basic flowing model. Based on the point-source solutions, the pressure responses of different well types in shale gas reservoirs with different configurations of top, bottom and lateral boundaries are obtained analytically or semi-analytically. Type curves are then plotted with Stehfest numerical inversion algorithm, and the effects of parameters related to gas adsorption, reservoir properties and hydraulic fracturing are analyzed.(5) Some agreed conclusions can be drawn from the analysis of type curves of different well types with respect to different basic flowing models:The existence of adsorbed gas in shales mainly affects the shape of the pressure derivative curves during interporosity flowing period, and the combination of laminar flow and diffusive flow in shale matrix mainly has effect on the occurance time of the reflection of interporosity flowing period in the derivative curves.(6) Flux distributions in multiply-hydraulic fracturing horizontal wells as a function of time and space are calculated, and the effects of important parameters pertinent to hydraulic fracturing are discussed. Some suggestions related to the design of hydraulic fracturing are then proposed according to the discussion.(7) Field data is collected to verify the validity and utility of the theoretical models proposed in the dissertation.The theoretical models presented here can be used to interpret pressure signals more accurately for shale gas reservoirs and thus have an important guidance for shale gas development.