No abstract

The Jurassic to Cretaceous sedimentary rocks of the Surat Basin in southeast Queensland host a significant volume of coal seam gas resources. Consequently, knowledge of the in situ stress is important for coal permeability enhancement and wellbore stability. Using wireline log data and direct stress measurements, we have calculated stress orientations from 36 wells and stress magnitudes from 7 wells across the Surat Basin. Our results reveal a relationship between high tectonic stress and proximity to structures within the underlying “basement” rocks. The influence of tectonic stresses is diminished with depth in areas with thicker sedimentary cover that are relatively far from the basement structures. We suggest that this relationship is due to the redistribution of in situ stresses around areas where basement is shallower and where basement structures, such as the Leichhardt‐Burunga Fault System, are present. This behavior is explained by a lower rigidity in the thickest basin cover, which reduces the ability to maintain higher tectonic stress. Over the entire Surat Basin, a significant amount of variability in in situ stress orientation is observed. The authors attribute this stress variability to complex plate boundary interactions on the northern and eastern margins of the Indo‐Australian Plate.

Secondary fractures and faults associated with larger, reservoir scale faults affect both permeability and permeability anisotropy and hence may play an important role in controlling the production behavior of a faulted reservoir. It is well known from geologic studies that there is a concentration of secondary fractures and faults in a damage zone adjacent to larger faults. Because there is usually inadequate data to incorporate damage zone fractures and faults into reservoir simulation models, in this study we utilize the principles of dynamic rupture propagation from earthquake seismology to predict the nature of fractured/damage zones associated with reservoir scale faults. We include geomechanical constraints in our reservoir model and propose a workflow to more routinely incorporate damage zones into reservoir simulation models. The model we propose calculates the extent of the damage zone along the fault plane by estimating the stress perturbation associated with dynamic rupture propagation. Fractures created by the stress pulse accompanying rupture propagation enhance permeability along reservoir scale faults in both the horizontal and vertical directions. We calibrate our modeling with observations from a number of studies and show that dynamic rupture propagation gives a reasonable first order approximation of damage zones in terms of permeability and permeability anisotropy in order to be incorporated into reservoir simulators.

Secondary fractures and faults associated with larger, reservoir scale faults affect both permeability and permeability anisotropy and hence may play an important role in controlling the production behavior of a faulted reservoir. It is well known from geologic studies that there is a concentration of secondary fractures and faults in a damage zone adjacent to larger faults. Because there is usually inadequate data to incorporate damage zone fractures and faults into reservoir simulation models, in this study we utilize the principles of dynamic rupture propagation from earthquake seismology to predict the nature of fractured/damage zones associated with reservoir scale faults. We include geomechanical constraints in our reservoir model and propose a workflow to more routinely incorporate damage zones into reservoir simulation models. The model we propose calculates the extent of the damage zone along the fault plane by estimating the stress perturbation associated with dynamic rupture propagation. Fractures created by the stress pulse accompanying rupture propagation enhance permeability along reservoir scale faults in both the horizontal and vertical directions. We calibrate our modeling with observations from a number of studies and show that dynamic rupture propagation gives a reasonable first order approximation of damage zones in terms of permeability and permeability anisotropy in order to be incorporated into reservoir simulators. Introduction Fractures present both problems and opportunities for exploration and production from hydrocarbon reservoirs. The heterogeneity and complexity of fluid flow paths in fractured rocks always makes it difficult to predict how to optimally produce a fractured reservoir. It is usually not possible to define the geometry of the fractures and faults controlling flow and it is difficult to integrate data from markedly different scales associated with faults mapped in seismic surveys and those seen in wellbore image logs. A number of studies in hydrogeology and the petroleum industry have dealt with modeling fractured reservoirs.1–4 Various methodologies, both deterministic and stochastic, have been developed to model reservoir heterogeneity on hydrocarbon flow and recovery. The work by Smart et al.5, Oda6–7, Maerten et al.8, Bourne and Willemse9, and Brown and Bruhn10 quantify the stress sensitivity of fractured reservoirs. Several studies11–13 that include fracture characterizations from wellbore images and fluid conductivity from the temperature and the production logs indicate fluid flow from critically stressed fractures. Additional studies emphasize the importance and challenges of coupling geomechanics in reservoir fluid flow.14–16 These studies found that geomechanical effects may be very significant in some of the fractured reservoirs. Secondary fractures and faults associated with larger scale faults appear to be quite important in controlling the permeability of some reservoirs. Densely concentrated secondary fractures and faults near larger faults are often referred to as damage zones, which are created at various stages of fault evolution: prior to faulting17–19, during fault growth20–25, and during the earthquake slip events25–28 along the existing faults associated with rupture propagation. Lockner et al.29 and Vermilye and Scholz23 show that the damage zones from the pre-faulting stage are very narrow and can be ignored for reservoir scale faults. The damage zone formed during fault growth can be modeled using dynamic rupture propagation along a fault plane30–33. In this paper, we first introduce a reservoir in which there appears to be significant permeability anisotropy associated with flow parallel to large reservoir scale faults. Next, we build a geomechanical model of the field and then discuss the relationship between fluid flow and geomechanics at well scale fracture and fault systems. To consider what happens in the reservoir at larger scale, we will utilize dynamic rupture modeling to theoretically predict the size and extent of damage zones associated with the reservoir scale faults. Finally, we utilize fine scale fluid flow simulations to illustrate the effects of these damage zones on permeability and permeability anisotropy of the reservoir. In contrast to static dislocation models due to slip events, which demonstrates the damage effects only at the tip of the existing faults, dynamic rupture propagation technique defines the damage zone all along the fault.

The contribution investigates the relationship between in situ stress regimes, natural fracture systems and the propagation of induced hydraulic fractures in APLNG's (Australia Pacific Liquid Natural Gas) acreage within the Jurassic to Cretaceous Surat Basin in southeast Queensland. On a regional scale the data suggest that large basement fault systems have significant influence on the lateral and vertical interplay between geomechanical components which ultimately control permeability distribution in the area. At a local scale we show several case studies of significant in-situ stress variations (changes in tectonic regime from reverse to strike-slip, changes in horizontal stress orientation as well as changes in differential horizontal stress magnitude) which are identified from wireline image log interpretations and geomechanical models constructed from wireline sonic and density data. These variations are reflected in hydraulic fracture propagation, which is monitored through microseismic monitoring, tiltmeter monitoring. Reverse stress regimes result in the propagation of horizontal fractures; in areas of higher differential stress linear hydraulic fracture orientations are common, whereas in regions of lower differential stress the orientation of hydraulic fractures appears influenced by both stress and pre-existing fractures. The paper is relevant for fracture simulation in areas with complex in-situ stress regimes. The major technical contribution of the study is the use of geomechanical modelling for predicting hydraulic fracture propagation styles.