Wave Propagation Models Capture Pressure Diffusion Behavior in Heterogeneous Unconventional Reservoirs
Written By: Hanyu Li
Hanyu Li is a first-year Masters student at Texas A&M University. He has recently been elected to serve as the 2016 – 2017 Graduate Representative for the Texas A&M Chapter of the Society of Petroleum Engineers.
Pressure Transient Analysis (PTA) and Rate Transient Analysis (RTA) are two important tools used to analyze the reservoir. PTA uses the pressure data obtained at the well to estimate important reservoir properties such as porosity and permeability. RTA on the other hand uses the rate data obtained at the well to estimate reservoir volume and ultimate recovery. However, it requires the reservoir to be in the boundary-dominated flow regime.
Estimating ultimate recovery using information from the boundary-dominated flow regime, which occurs when the pressure diffusion from the well has reached all the boundaries in the reservoir, is not practically achievable in unconventional (low permeability) resources since the time required to reach this flow regime is quite large (typically years). In response to this problem, the ultimate objective of this research project is to estimate ultimate recovery of horizontal wells with multistage fractures using only information obtained after the pressure diffusion from adjacent fracture stages start to interfere with each other. Fracture interference is an equivalent boundary effect that can be achieved much faster than boundary-dominated flow (typically in days). We have taken the first step towards achieving this objective by simulating the boundary condition in heterogeneous reservoirs using the similarity between pressure diffusion and electromagnetic and ray wave propagation. Estimating ultimate recovery more accurately will enable us to optimize well spacing and reduce the number of unnecessary wells drilled, saving capital investments of many millions of dollars.
We have implemented the mechanism of electromagnetic and ray wave propagation in pressure diffusion models to simulate boundary conditions that affect flow in the reservoir. When a light wave reaches the interface of two media, some light is reflected back to the same media while the remainder is transmitted into the other media. A pressure diffusion behaves in a similar way when it reaches a boundary in the reservoir. Therefore, we have incorporated reflection and transmission coefficients into the drawdown solution for pressure diffusion. We used a channel reservoir to validate our approach. The pressure transient analysis solution obtained using our method matches well with the true solution generated by the pressure transient software Ecrin. We can use the accurate early time match to estimate important reservoir properties including porosity and permeability. The accurate late time pressure information can be transformed into rate information and then used to estimate ultimate recovery.