Carbonate Rock Typing: Challenges, Mitigations and Pragmatic Workflows

Abstract

Abstract Carbonate rock typing is a fundamental to capture the variation of several static, dynamic petrophysical properties driven by depositional and diagenetic overprint, thus, is an important input for controlling the reservoir properties. Since rock undergoes varying degree of diagenesis, therefore, establishing a simple relationship of geology with petrophysics is difficult. This paper highlights challenges, mitigations, quality checks, while defining carbonate rock types from core to log domain, preserving desired heterogeneity through robust schemes and workflows. The fundamental to rock typing relies upon measuring all desired discriminating geology parameters (texture, cementation, dissolution, pore type, grain size, grain sorting etc.), petrophysical parameters (Phi, K, ρma, PTR, FRF, Pc) and log responses (quad-combo, NMR, Images, Elemental Yields etc.) at a common depth, with regular interval capturing all facies. Removing outliers and harmonizing core geological-petrophysical measurements and logs is a key. The rock-typing workflow generates Static Rock Types (SRT) in core domain by integrating Petrophysical Groups (PG) with Candidate Rock Type (CRT), then propagating to 1D log domain in all wells and finally distributing in 3D space using geological concept. Several data synthesis steps, challenges and quality checks while integrating PGs, CRTs to define SRTs in core and log domain shall be presented. This rock typing approach is pragmatic for the fact that it: (a) defines rock types by capturing geological processes and integrating with static and dynamic petrophysics from core and log scales; and (b) provides a systematic approach from core to 3D field model for variable data scale and scenarios; (c) can be updated easily with enhancement in data quantity and quality during different phases of reservoir and field development. Thus, each SRT can be considered as geologically driven rock type with unique petrophysical properties like Phi, K, Pc, PTR etc. to construct reliable Saturation Height Function (SHF) Model to determine initial water saturation representing the primary drainage cycle to determine in-place hydrocarbon volumes. Disparity in any step of core and log data QC, data correction and integration between geology and petrophysics to define SRTs, would lead into inconsistency between initial water saturation (Sw) from logs, core and SHF Model. Finally, all derived geological and petrophysical variable and its uncertainties including rock types, porosity, permeability and Sw shall be captured to build reliable static and dynamic models. This novel, and versatile scheme of Carbonate Rock Typing backed up by strong quality control and systematic technical workflows and assurances has been deployed in several simple to complex carbonate reservoirs in Onshore fields of Abu Dhabi. Accordingly, the Static Model is more representative to true reservoir characterization providing better prediction from Dynamic Models, supporting field development plans through IOR & EOR Schemes.