Mechanisms for Huff-n-Puff Cyclic Gas Injection into Unconventional Reservoirs

Abstract

Unconventional oil reservoirs such as the Eagle Ford have had tremendous success over the last decade, but challenges remain as flow rates drop quickly and recovery factors are low; thus, enhanced oil recovery methods are needed to increase recovery. Interest in cyclic gas injection has risen as a number of successful pilots have been reported; however, little information is available on recovery mechanisms for the process. This paper evaluates oil swelling caused by diffusion and advection processes for gas injection in unconventional reservoirs. To accurately evaluate gas penetration into the matrix, the surface area of the hydraulic fractures needs to be known, and in this work, three different methods are used to estimate the area: volumetrics, well flow rates and linear fluid flow equations. Fick's law is used to determine the gas penetration depth caused by diffusion, and the linear form of Darcy's law is used to find the amount from advection. Then, with the use of swelling test information from lab tests, we are able to approximate the amount of oil recovery expected from cyclic gas injection operations. During the gas injection phase, gas from the fractures can enter the matrix by both advection (Darcy driven flow) and diffusion. We estimate that over 200 million scf of gas can enter the matrix during a 100 day injection/soak period. Using typical reservoir and fluid parameters, it appears that 40% is due to diffusion and 60% is due to advection. Sensitivity analysis shows that these numbers vary considerable based on the parameters used. Analytical models also show that during a 100 day production timeframe, over 14,000 stock tank barrels (STB) of oil can be produced due to huff-n-puff gas injection. Both gas injection and oil recovery amounts are compared to recent Eagle Ford gas injection pilot data, and the model results are consistent with the field pilot data. By determining the relative importance of the different recovery mechanisms, this paper provides a better understanding of what is happening in unconventional reservoirs during cyclic gas injection. This will allow more efficient injection schemes to be designed in the future.

Modeling Interwell Fracture Interference and Huff-N-Puff Pressure Containment in Eagle Ford Using EDFM

“…For both systems, (a) and (b), PSS will be obtained. However, the only difference is that (a) will exhibit a transition flow regime (between transient and PSS), which is a function of the different sizes and shapes of the matrix blocks (Hoffman, 2019). is performed based on the fracturing design plan on the left and the fracture identification from WBI on the right.…”

“…Darcy's law) and diffusion (Fick's Law). Over the last decade, numerous studies have been dedicated to Huff-n-Puff( Wan et al, 2013;Chen et al,.2014; Yu et al, 2014; Zu et al., 2015;Hoffman and Evans, 2016;Kanfar et al, 2017;Alharthy et al, 2018;Hoffman and Rutledge, 2019). In fact, in Eagle Ford, Huff-n-Puff pilots have also exhibited tremendous success over the last decade, but challenges remain as flow rates drop quickly and recovery factors are low(Hoffman and Evans, 2019).…”

Recovery Mechanisms for Cyclic (Huff-n-Puff) Gas Injection in Unconventional Reservoirs: A Quantitative Evaluation Using Numerical Simulation

“…While hydraulic fractures are often considered to have complexity with fractures occurring in multiple directions, for gas injection, the fracture surface area in contact with the matrix is one of the most important attributes for recovering additional hydrocarbons. In this model, simple planar fractures are used that have the same surface area as the more complex ones; thus, they can be considered as reasonable approximations [20]. For this model, the total surface area of the fractures is 3.7 million ft 2 .…”

“…Hamdi et al [19] discussed the complex function of mass transfer and extraction processes related to a number of different potential mechanisms. Hoffman and Rutledge [20] compared the role of diffusion and advection for the oil-swelling mechanism using analytical models. While these are good papers to start discussing mechanisms, they either focus on a single mechanism or have only a qualitative comparison of different mechanisms.…”

Review on Phase Behavior in Tight Porous Media and Microscopic Flow Mechanism of CO2 Huff-n-Puff in Tight Oil Reservoirs

“…Tight oil reservoirs have been considered as the most promising resources among unconventional oil reservoirs. At present, the main method to exploit tight oil reservoirs is primary depletion after massive hydraulic fracturing along long horizontal wells, but the primary recovery factor is only 5%~10% [6][7][8][9][10]. Successful development of tight oil reservoirs has been achieved in North America (e.g., Bakken Basin, Permian Basin, and Eagle Ford) by using CO 2 huff-n-puff [11][12][13][14][15][16][17].…”