Capillary seal capacity of faults under hydrodynamic conditions

Abstract

Many fault bound traps are underfilled despite the top seal capacity being secure. The hydrocarbon sealing performance of faults themselves can be compromised either by mechanical or capillary process. Capillary process can be important either due to juxtaposition or to fine-grained clay or cataclastic material within the fault zone itself. There is debate about how important each of these mechanisms is over geological timescales of hydrocarbon trapping. Recent work has provided insights into fine-tuning capillary-related fault seal calibration methodologies. Over the last 15 years, vigorous scientific debate with multiple published laboratory experiments and modelling studies has led some researchers and industry technologists to theorise that for water-wet conventional hydrocarbon reservoirs, the relative water permeability in the reservoir (towards the top of the hydrocarbon column) may become very small, but in practice never reach zero. While not advocating for either side in this debate, the importance of accounting for hydrodynamic conditions regardless of the capillary sealing mechanism is demonstrated. Additionally, it is noted that nonzero relative water permeability has implications on how a seal's capillary threshold pressure for the nonwetting hydrocarbon phase is estimated from field data. In the particular case where there are pressure differences between unproduced hydrocarbon reservoirs on either side of a fault, then the hydrocarbon saturation must be discontinuous across the fault. For hydrocarbon leakage to occur across the entire thickness of the fault zone, the hydrocarbon pressure must exceed the threshold pressure on the side of the fault zone with the highest formation water hydraulic head. This approach to estimating across-fault pressure difference will result in an improved calibration data set used for predrill estimation of capillary fault seal capacity.

Seal failure assessment of a major gas field via integration of seal properties and leakage phenomena

“…For failure of the topseal in shear mode, it is conceivable that much lower values of gas pressure could have induced shear failure of the pre-existing faults and fractures in the topseal, and in turn led to leakage from a number of fault-related loci. It is difficult to assess this possibility for the Scarborough Gas Field (see Underschultz and Strand, 2016 for theoretical considerations of pressure induced reactivation of critically stressed faults) because there are no cores calibrating the shear strength of the fractured Muderong Formation and in situ stress conditions are also uncalibrated. The fact that a polygonal fault system is pervasively developed within the seal and is clearly imaged by the seismic perhaps indicates the presence of a network of sub-seismic fractures too (Cartwright, 2011), and it is certainly a possibility that some subset of these fractures might have favorable orientations to be critically stressed in the presentday stress field.…”

Field Surveys in Heterogeneous Rock Masses Aimed at Hydraulic Conductivity Assessment

“…Guidelines to de ne groundwater models at regional scale were developed by many authors, suggesting appropriate methodologies and procedures for fault zone parameterization and representation (Turnadge et al 2018; Underschultz et al 2018). A general model conceptualization (Bense et al 2013) considers the total width of a fault zone made up of two components: the fault core (FC) and the damage zone (DZ).…”

Structural characteristics of transtensional fault system and its implication for hydrocarbon accumulation in S Block, South Asia area

“…The transtensional fault concept was first proposed in the 1970s (Hardinget al1979), and the strike-slip twisting was introduced to explain the evolution of hydrocarbon traps, basins and orogenic belts. At present, research about the causes of the tectonic pattern, assemblage types and relationship with oil and gas accumulation is abundant, which effectively promote the oil and gas exploration around the world (Castro et al 2015;Li et al 2016;Audrey et al 2017;Robsonet al2018;Feng et al 2018;Nabavi et al 2019;Shaun et al 2016;Underschultz et al 2016;Wuet al 2020;Yu et al 2014;Qin et al 2017;. It is found that the S Block in the southern area of the Indus Basin is the main oil exploration basin in South Asia.…”