Late Time Response Analysis in Pressure Build Up Tests: Challenges and Solutions

One of the most important features that impacts the facility size and production rate commitment is the minimum connected volume which can be explored through pressure Build Up and any misinterpretation, costs the operators significantly. This paper aims to address the challenges in Late Time Response analysis of pressure Build up well tests that exploits the reservoir boundary. Knowing that the well test response at late time shows the fingerprint of multiple features such as aquifer, sand discontinuity, baffles, faults, boundaries, etc., it will be challenging to quantify the extends of the reservoir using convolved late response. Two main numerical models were built for oil and gas reservoirs in an elongated reservoir to study the lateral boundary features such as closed boundary and aquifer and understand the effect of these features on late time response to get a clearer response. Using two models, the impact of mobility contrast between the hydrocarbon, boundary distance, aquifer strength, boundary movement and structure complexity was simulated. The major finding of this paper is that a unit slope straight line can be seen in buildup test which doesn't fit any classical and analytical model for pressure build up as the expectation is to have negative slope and down-ward hump for build up response of closed boundary and aquifer. However, this can be explained through mobility reduction from hydrocarbon (oil/gas) to water. The simulation time (build up test duration) was extended to see the long-term impact theoretically and at the end the same expectation was confirmed by simulation. After radial flow, channel flow (half slope) and then unit slope (mobility reduction) and then at last the drop in the derivative response is observed. However, the drop as signature of aquifer or limited volume happened beyond practical test duration (> 1000 hours). The work highlights the true workflow to identify the true response from reservoir and have reliable reservoir characterization and shows the case studies that address the cost on 1) misinterpretation of late time response and 2) excluding the well test from analysis due to its complexity to highlight the value and criticality of the integrated analysis.