Francisco D. Tovar

Experimental Investigation of Enhanced Recovery in Unconventional Liquid Reservoirs using CO2: A Look Ahead to the Future of Unconventional EOR

The poor rock quality and matrix permeability several orders of magnitude lower than conventional oil reservoirs observed in unconventional liquid reservoirs (ULR) presents many uncertainties on the storage capacity of the rock and the possibility of enhancing recovery. The technological advances in multiple stage hydraulic fracturing and horizontal drilling have improved the overall profitability of oil shale plays by enhancing the matrix – wellbore connectivity. The combination of these technologies has become the key factor for the operators to reach economically attractive production rates in the exploitation of ULR, causing a lot of focus on their improvement. However, as the reservoir matures, primary production mechanisms no longer drive oil to the hydraulic fractures, making the improvement of matrix – wellbore connectivity insufficient to provide economically attractive production rates. Therefore, the need to develop enhanced recovery techniques in order to improve the displacement of the oil from the matrix, maintain profitable production rates, extend the life of the assets and increase ultimate oil recovery becomes evident. This study presents experimental results on the use of CO2 as an enhanced oil recovery (EOR) agent in preserved, rotary sidewall reservoir core samples with negligible permeability. To simulate the presence of hydraulic fractures, the ULR cores were surrounded by high permeability glass beads and packed in a core holder. The high permeability media was then saturated with CO2 at constant pressure and temperature during the experiment. Production was monitored and the experiment was imaged using x-ray computed tomography to track saturation changes inside the core samples. The results of this investigation support CO2 as a promising EOR agent for ULR. Oil recovery was estimated to be between 18 to 55% of OOIP. We provide a detailed description of the experimental set up and procedures. The analysis of the x-ray computed tomography images revealed saturation changes within the ULR core as a result of CO2 injection. A discussion about the mechanisms is presented, including diffusion and reduction in capillary forces. This paper opens a door to the investigation of CO2 enhanced oil recovery in ULR.

Horizontal Hydraulic Fracture Design for Optimal Well Productivity in Anisotropic Reservoirs with Different Aspect Ratios

Summary The economic feasibility of the exploitation of unconventional resources is highly dependent on the ability of the operator to maximize individual well productivity, making hydraulic fracture design and implementation the defining factor for a successful field development in most cases. Some unconventional reservoirs, as shallow coal bed methane and over-pressured oil and gas shale formations, commonly present the minimum principal stress in the vertical direction, resulting in the occurrence of horizontal hydraulic fractures. Models for the transient flow and pressure behavior of horizontal fractures emanating from vertical wells exist and clearly show distinct performance from those for vertical fractures. This suggests that the widely accepted unified fracture design (UFD) approach to maximize well productivity for vertical and horizontal wells with vertical hydraulic fractures cannot be used for horizontal fractures. Thereafter, the necessity for guidelines to model and design horizontal fractures becomes evident. This investigation begins by presenting a new set of equations for horizontal fracture design based on the UFD approach, which allows the direct calculation of fracture width, half-length and conductivity for a given proppant number. Later, a reservoir numerical simulator is used to model well productivity behavior for horizontal fractures in homogeneous formations, with or without vertical to horizontal permeability anisotropy and for different aspect ratios as a function of suitably-defined proppant number, dimensionless fracture conductivity, and fracture penetration index parameters. The findings of this work reveal a complex behavior for horizontal fractures that prohibits the extrapolation of previous generalizations between proppant number, penetration index and dimensionless fracture conductivity established for vertical fractures. For a number of scenarios, new relationships among these variables are provided to guide horizontal fracture design. Anisotropy and reservoir aspect ratio were also found to significantly impact fracture performance. Additionally, a set of multi-variable functions that permit the estimation of maximum achievable productivity index for the horizontal fracture has been fitted, based on commonly known reservoir parameters and the proppant number. This investigation provides a comprehensive framework to assist the design of optimal horizontal fracture geometry that maximizes productivity for a given mass of proppant.