This paper presents a unique workflow for gas reserves evaluation in fields with commingled production from several low permeability reservoirs. The workflow was originally developed for gas reserves evaluation of the Lower Vicksburg (LV) sands and the paper illustrates the key steps in the methodology. Developing Lower Vicksburg sands has been a great challenge to all operators in the region, not only because of the high drilling and completion cost, but also due to the high risk and uncertainty involved in the process. To make wise investments in such a difficult environment, it is crucial to understand the real value of the remaining reserves.
The outcome of a reserves evaluation depends on the amount and quality of the data, the knowledge and experience of the evaluators, and the methodology and workflow used during the evaluation process. Although we deal with dry gas reservoirs, the challenge lies in the difficulty of solving relatively simple equations that result from a combination of complicated reservoir and production characteristics. As a result, a wide range of answers may be obtained from the same data set and frequent revision of the reserves number becomes unavoidable. However, it has been our experience that the accuracy can be greatly improved through sound engineering practice and workflow. This paper illustrates a robust process to quantify the reserves in the LV reservoirs. This unique workflow includes sound data management and consistent analysis processes that allow for iterations among different methodologies aimed at minimizing the uncertainty and improving accuracy in the results. The paper also highlights some common pitfalls and mistakes normally made today in the reserves evaluation process for low permeability reservoirs, which should prove highly beneficial to the petroleum industry. In fact, the workflow would be useful toward obtaining correct reserves estimations for virtually any low permeability gas field where production from several horizons are commingled without the need of a full field reservoir simulation.
In this paper, we will use a field reserves study as an example to illustrate our unique workflow. The objective was to estimate remaining reserves in the LV reservoirs. The scope of the study includes six Lower Vicksburg sands (namely A sand, B sand, C sand, D sand, E sand, and F sand) found in less than 100 wells. It needs to be pointed out here that any analysis results presented in this paper do not represent the results from the real study. These are strictly for demonstrating the workflow process.
The Lower Vicksburg sands in the study area are heterogeneous, low permeability, and geo-pressured. Economics dictate the need for hydraulically fractured completions and commingled production from multiple zones, which complicates the production data analysis. Due to the low permeability, the average reservoir pressure cannot be accurately determined from well shut-in pressures. Fig. 1 shows an example of measured shut-in pressures in different LV reservoirs. Many wells experienced sanding and casing collapse problems due to compaction during production, which resulted in lower gas recovery. Well histories are quite complicated due to the workovers (add or squeeze perforations, refracs, commingling, etc.). Fig. 2 shows a typical well production history in this project. It becomes a real challenge to allocate production by the reservoirs.