This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 208321, “Effect of Errors in Initial Pressure Measurement on Rate-Transient Analysis in Unconventional Reservoirs,” by Himanshu S. Jha, SPE, Texas A&M University; Aaditya Khanal, The University of Texas at Tyler; and W. John Lee, SPE, Texas A&M University. The paper has not been peer reviewed.

This paper presents a mathematical analysis of how incorrect estimates of initial reservoir pressure (pi) may affect rate-transient analysis (RTA) in ultralow-permeability reservoirs. Measured values of pi are rarely available, and the value estimated using alternative approaches, such as a diagnostic fracture injection test (DFIT), can be time-consuming and expensive and also may be inaccurate. In the complete paper, the authors examine what can and cannot be achieved with accuracy when reliable estimates of pi are not available.

Introduction

RTA and production forecasting using historical production data require flow-regime identification for model parameter calculation. The task of flow-regime identification is best achieved under constant-rate flowing conditions. However, field conditions and operational changes almost always cause flowing bottomhole pressure (BHP) to change during production. Use of rate-normalized pressure (RNP) can help mitigate the effect of changing BHP under smoothly changing conditions. Calculation of RNP requires an accurate estimate of pi, which often is unavailable.

The authors’ work shows that the slope of a log-log plot of RNP vs. time or material balance time (MBT) is relatively insensitive to the value of pi. However, a Cartesian plot of RNP vs. timen (where n may be 0.5 for transient linear flow or a different value for more general “power-law behavior”) rotates the expected straight line for different values of pi or used in calculation of RNP. Therefore, the task of flow-regime identification can be achieved with reasonable certainty with only an estimate of initial pressure, but analysis with RNP vs. timen Cartesian plots may be unreliable.

Keywords:
drillstem/well testing, hydraulic fracturing, production control, drillstem testing, calculation, pressure transient testing, production monitoring, formation evaluation, upstream oil & gas, reservoir surveillance
Subjects:
Drillstem/well testing, Formation Evaluation & Management, Hydraulic Fracturing, Pressure transient analysis, Reservoir Characterization, Well & Reservoir Surveillance and Monitoring
This content is only available via PDF.
Copyright 2021, Unconventional Resources Technology Conference
You can access this article if you purchase or spend a download.