Complex Reservoir Characterization of Ultra Tight Naturally Fractured Carbonate Over-Pressured Gas Condensate Reservoir Using Dynamic Data Analysis
Authors: Hussain Sadam , Ahmed Zaheer , Mehmood Saad , Azeem Abdul , Iqbal Muneez , Zakir Mufaddal Murtaza , Ejaz Muhtashim , Wu Xingru 
Published: February 2024

Abstract
Ultra-Tight fractured carbonate reservoirs (UTFCR) are the world's most geologically complex and heterogeneous reservoirs. To develop over-pressured UTFCR, reservoir dynamics and boundaries must be characterized using measurements and interpretations on various scales. Pressure/rate transient analysis (PTA & RTA) can be performed to characterize UTFCR on a relatively large scale for field development. The study aims to apply dynamic data interpretation to characterize reservoir and boundaries, estimate gas initial in-place (GIIP), and evaluate well spacing for field development.
We used three pressure buildup (PBU) tests and two-and-a-half years of production data to characterize the reservoir system and boundaries with varying models. Data quality assurance/quality control (QA/QC) and synchronization were also performed before PBU analysis. For RTA, production rates and flowing wellhead pressure (FWHP) history of two and a half years were used in the analysis. Before quantitative analysis and reservoir characterization, we converted FWHP to bottom-hole flowing pressure (BHFP). Data was also filtered during RTA to select a pseudo-steady state for analysis. Complex boundaries were characterized using PTA and RTA for GIIP and drainage area for field development planning. Also, initial pressure (Pi) was estimated, which was previously unknown. Sensitivity analysis was performed to evaluate uncertain parameters.
Due to significant uncertainties in the reservoir characterization from geological study, different reservoir and boundary models were matched with PBUs. It was observed that wellbore storage was improved during the second and third PBU, which may be due to a decrease in natural fractures (N.F.s) permeability/aperture. Additionally, fracture and matrix weighted-average permeability decreased from 3.6 md to 2 md. Fracture half-length (Xf) was also reduced from 400ft to 190ft during the second and third PBUs, indicating healing of N.F.s due to reservoir compaction in the reservoir depletion process. Furthermore, the reservoir system changed from a dual-porosity to a single-porosity system. Initially, reservoir boundaries were recognized as a closed system. However, at a very late time, the analysis showed an increase in flow capacity far away from the well, and the multilinear composite model matched with all PBUs, indicating that the reservoir is produced from three composites/compartments having different mobilities, diffusivities, and sizes. Also, the estimated Pi based on PTA and RTA was in close agreement with Pi estimated based on drilling information. Based on PBU analysis, the estimated GIIP was ~30Bscf and a ~2km drainage radius. On the other hand, RTA matched with a dual-porosity close reservoir system having lower weighted-average fracture and matrix permeability (1.1md). Xf of ~2300ft was estimated; the N.F. network seems pervasive. RTA estimated GIIP ranged between 25-45 Bscf and drainage radius between 2 – 2.3km.
A conventional reservoir engineering approach for field development cannot be used to characterize UTFCR. This study will assist and provide a workflow to characterize UTFCR and estimate GIIP/drainage area for field development using dynamic data information. It can be concluded that UTFCR can vary from Type I to III fractured reservoirs and is produced from multi-composites/compartments. Field appraisal and development should be carefully planned to avoid interference and natural fracture-hit.