Methodology to Estimate the Optimal Production Rate and OWC Advance in Naturally Fractured Reservoirs in the South of Mexico

摘要

A great problem of naturally fractured reservoirs is the abrupt irruption of water in wells. To predict this behavior, a methodology that uses numerical sector models and discrete fracture Network (DFN) was designed; so-called Pseudoradial models (PRM). These models were calibrated to match the water invaded wells production history and subsequently it was applied to proposed or new wells. This methodology was used successfully in Pijije field, Mexico. The first pseudoradial models were generated in the Pijije field due to the abrupt water irruption problem it presented. Three well models were built based on the calibration of the model built for the Pijije-101 well. The DFN was generated from average characterization of fractures obtained from FMI logs and general input from the field pressure and production behavior. Once the variables of uncertainty were analyzed, a conventional radial model was initially built and calibrated with the historical production and pressure profiles. To achieve this match, the values of porosity in both systems (matrix and fracture) were changed drastically and the water front advance resulting was homogeneous and only present a coning phenomenon when the OWC reaches a minimum distance of 200 meters from the well. The second part of the analysis was carried out with the pseudoradial model; in this case, the parameters with greater impact were the characteristics of discrete fracture network. The pseudoradial model managed to reproduce the well production and pressure history. Analyzing the results, it was concluded that both scenarios were acceptable. Reviewing the water advance fronts in both cases, the pseudoradial model did not have a uniform front of OWC advance and showed areas not drained (corresponding to present or absence of fractures) and the water moves through the preferential fracture channels represented by the DFN in the model. Two methods for determining critical rates or optimal production rates (OPR) and the OWC advance were evaluated: conventional radial models and pseudoradial models. The study was carried out to the Pijije-101 and Terra JSK field wells that presented abrupt water irruption possibly associated with water conning in the producing formation. Later the methodology was applied to new wells in the same field for calculating their optimal production rates. Although conventional radial model does not consider features and phenomena present in NFR, the movement of fluids and conning effect was correctly model. The pseudoradial model, in the same way, reproduced the dynamic behavior observed in this conventional case but presenting irregular advance of the OWC and areas not drained or partially drained which according to literature is more realistic. The main difference between the two models is the distribution of permeability and how the fluids move within the reservoir. Conventional radial model presents only a vertical variation, while a tensor of permeability (i, j, k) was included in the pseudoradial models. Due to the complexity of the NFR, it is recommended to perform analysis of water conning using pseudoradial models since it manages to represent the fractured medium with a more realistic and physical sense of the dynamic involved.