Interpretation of mini-frac and flowback pressure response: application to unconventional reservoirs in the UAE

Abstract

The main objective of the thesis was to develop a working knowledge of the underlying concepts for developing unconventional shale in the UAE Diyab formation. To achieve this objective, I identified four broad subsets as listed: (1) Reservoir engineering evaluation of the UAE Diyab (Upper Jurassic, gas condensate) and Shilaif (Middle Cretaceous, light oil) unconventional shale development. (2) Conduct laboratory experiments in Diyab cores to determine benchtop permeability of cores with and without fractures. (3) Understand the mini-frac pressure fall-off analysis as the major method for determining in-situ matrix permeability for use in reservoir evaluation, modeling, and forecasting performance of stimulated shale reservoirs. (4) Determine permeability enhancement in a Diyab stimulated well using rate transient analysis (RTA). This permeability is the effective permeability composed of matrix rock permeability and microfracture permeability of the stimulated reservoir section. In regard to reservoir evaluation, I constructed a compositional reservoir model of Shilaif light oil in a small sector surrounding an exploration well. To obtain the stimulated reservoir permeability, and permeability of imbedded fracture system, I performed rate transient analysis (RTA) using the Shilaif exploration well production data. Finally, I used this permeability in the compositional model of the reservoir to forecast the well’s future performance. In regard to laboratory experiments, I measured permeability of fractured and unfractured core samples from Diyab. Finally, much of my time was spent on evaluating the mini-frac pressure fall-off theory, and determining the effect of hydraulic fracture filtrate on the magnitude of the stress changes near the two surfaces of the hydraulic fracture which provided information about the extent of micro-fracture creation and re-stimulation. Among the four objectives, evaluation of the mini-frac theory and its interpretation consumed most of my research effort. Mini-frac injection tests, commonly known as Diagnostic Fracture Injection Test (DFIT), are of great value in determining the minimum horizontal stress and permeability of the matrix rock under reservoir conditions. This permeability can be compared with the permeability of core samples from the same formation to determine how closely laboratory-measured permeabilities reflect the formation permeability under reservoir stress conditions. In this thesis, (1) I present both analytical and numerical modeling of single- and two-phase flow in support of the interpretation of the pressure falloff from field DFIT data, and (2) I analyze the pressure falloff data of a laboratory conducted DFIT in a granite core by Luke Frash (Ph.D. Thesis, CSM, 2014). I applied my interpretative procedures used on the laboratory DFIT data to a mini-frac test from Diyab formation. Finally, I determined the depth of filtrate invasion and depth of formation cooling. I used the quantitative information of filtrate invasion, formation cooling, and rock deformation at fracture surface to determine the net stress change near the surface of the fracture, which is commonly referred to as the ‘stress shadow’ effect.