Characterizing Hydraulic Fractures in Shale Gas Reservoirs Using Transient Pressure Tests

Hydraulic fracturing combined with horizontal drilling has been the technology that makes it possible to economically produce natural gas from unconventional shale gas or tight gas reservoirs. Hydraulic fracturing operations, in particular, multistage fracturing treatments along with horizontal wells in unconventional formations create complex fracture geometries or networks, which are difficult to characterize. The traditional analysis using a single vertical or horizontal fracture concept may be no longer applicable. Knowledge of these created fracture properties, such as their spatial distribution, extension and fracture areas, is essential information to evaluate stimulation results. However, there are currently few effective approaches available for quantifying hydraulic fractures in unconventional reservoirs. This work presents an unconventional gas reservoir simulator and its application to quantify hydraulic fractures in shale gas reservoirs using transient pressure data. The numerical model incorporates most known physical processes for gas production from unconventional reservoirs, including two-phase flow of liquid and gas, Klinkenberg effect, non-Darcy flow, and nonlinear adsorption. In addition, the model is able to handle various types and scales of fractures or heterogeneity using continuum, discrete or hybrid modeling approaches under different well production conditions of varying rate or pressure. Our modeling studies indicate that the most sensitive parameter of hydraulic fractures to early transient gas flow through extremely low permeability rock is actually the fracture-matrix contacting area, generated by fracturing stimulation. Based on this observation, it is possible to use transient pressure testing data to estimate the area of fractures generated from fracturing operations. We will conduct a series of modeling studies and present a methodology using typical transient pressure responses, simulated by the numerical model, to estimate fracture areas created or to quantity hydraulic fractures with traditional well testing technology. The type curves of pressure transients from this study can be used for quantify hydraulic fractures in field application. Introduction For the unconventional gas reservoirs, hydraulic fractures characterization is important in assuring the maximum stimulation efficiency (Baree et al. 2009; Apaydin et al. 2012; Miskimins, 2009). A lot of research has been carried out on the flow behavior analysis of vertical wells with finite-conductivity or infinite-conductivity hydraulic fractures. Cinco-ley and Samaniego summarized fluids flow in a hydraulic fractured well could be divided into four periods: fracture linear flow, bilinear flow, formation linear flow and pseudo-radial flow (Cinco-ley and Samaniego, 1981). Nobakht and Clarkson pointed out the dominant flow regime observed in most fractured tight/shale gas wells is the third one, formation linear flow, which may continue for several years (Nobakht and Clarkson, 2012).