Identification of productive zones in unconventional reservoirs
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Large-scale multi-stage fracture treatments in long horizontal wells have enabled economic hydrocarbon production from source mudrocks. A productive zone in mudrocks is defined as a region with high production or high productive potential. Rock fracability is an important parameter used in evaluating the productive potential in a source mudrocks. The fracability of the rock is the degree to which hydraulic fracturing can create a dense and conductive fracture network upon fracturing in the formation. However, there is no agreement on the formation geomechanical properties that result in a source rock having good fracability. The objective of this thesis to identify formation properties that may be related to fracability and to identify how these properties may be assessed from well logs. Once the properties have been identified, data from 15 wells in the Barnett shale are used to assess the effect of the properties on long-term production. We performed a sensitivity study on the effect of formation properties on the size of the stimulated rock volume. Field-scale simulations of a single fracturing stage were performed with CFRAC (Complex Fracturing ReseArch Code), a fracture simulator that couples fluid flow and stresses induced by fracture deformation (sliding and opening) in large, discrete fracture networks. Two-hundred simulations were performed with a uniform space filling design: a low discrepancy quasi-random sequence uniformly filling the hyper-parameter space. Each simulation used a different stochastically generated natural fracture network even though each was statistically similar in terms of fracture orientation, density, and length. Simulation results were post-processed to estimate a measure of the stimulated reservoir volume in each simulation. Parameters affecting tendency for shear stimulation fracture conductivity had the biggest effect on the stimulated reservoir volume. Unfortunately, these parameters are not easy to estimate in-situ. A review of the literature was carried out to understand the relationship between unpropped fracture conductivity (which cannot easily be measured in-situ) and other formation properties that could be quantified with available techniques. We used the concept of shear dilation angle to describe increase in conductivity in response to sliding. The dilation angle can be correlated to the joint compressive strength of the rock which is equal to the unconfined compressive strength for an unaltered rock. Unconfined compressive strength can be estimated from sonic logs. This hypothesis was tested on 15 wells in the Barnett Shale. Hydrocarbon-bearing zones were identified in the wells using the gamma ray log and the cumulative mechanical properties of the zones were compared to the long-term production of the wells. Results show that including the unconfined compressive strength in finding productive zones will improve the effectiveness of prediction models. Such a behavior alludes to the possibility that properties affecting unpropped fracture conductivity should be given consideration while planning and implementing fracture treatments in unconventional plays.