Reserves Estimation and Production Forecasting Using a Quick Sector Modeling Technique in Tight Oil Sands – A Case Study

Abstract

Abstract This paper focuses on production forecasting of a tight oil reservoir using numerical sector modeling technique and its advantages over conventional decline curve analysis (DCA) and material balance methods. Sector modeling has resulted in better control over prediction as compared to the actual performance. This is mainly due to better understanding and handling of vertical reservoir heterogeneity and pore pressure depletion, where in, the later is treated as an implicit constant thereby falling short of capturing pore pressure changes and resulting impact over well rates and drainage areas. A well was drilled in the South Indus basin to explore, assess, and produce hydrocarbons from cretaceous age sandstone reservoirs. This field has a complex structure due to multiple splay faults. The well logs indicated hydrocarbon and perforations were carried but the well did not flow naturally due to tight nature. Hydraulic frac was executed and well was commissioned on Artificial lift and produced at optimized rate of ~180 Bopd. A pressure buildup survey was also carried out which indicated ~1 mD permeability. The objective of this study was to determine the reliable tool for prediction of tight oil fields and evaluate future development scenarios. Analytical approaches including conventional Material balance & DCA as well as numerical simulation technique was adopted for performance prediction. Therefore, material balance and DCA was performed based on available data, and subsequently, a sector model was developed using commercial simulator. Sufficient structure around the wellbore was imported in the static modeling package wherein the grid model was created having vertically varying layers based on interpreted well logs. All the available petrophysics, PVT, pressure and production data were incorporated in the sector model along with completion and history details. The numerical model predictions are more realistic as compared to conventional methods. Comparison with actual performance of the well for ~1 year shows that it is closely in agreement with prediction from numerical model whereas results from analytical methods were showing pessimistic forecast with significant offset from actual well rates. The calibration of numerical model with the actual performance provided confidence on the model for field development. To develop and optimally produce the field, various sensitivities were run on the model which concluded drilling of a new horizontal well in the structure with multi-stage fracs to be the best option for improving production rate and significant recovery from the field. This study concludes that for tight oil reservoirs, conventional analytical methods such as material balance or DCA do not provide adequate results in comparison to numerical reservoir simulation technique. Even simple grid model is best suited because it incorporates reservoir heterogeneities, important structural complexities, overall acreage and can better predict dynamic behavior of these reservoirs. This case study is unique in comparison with the conventional sector modelling in a way that it captures the structural shape, different petrophysical properties for each layer based on well logs & offset core data, and also involves the history match of available data. It also effectively captures the impact of hydraulic fracturing in the well, hence helps to create realistic production profiles.