Production-induced Compaction Research Articles

Finite Element Modeling of Production-Induced Compaction and Subsidence in a Reservoir along Coastal Louisiana

ABSTRACT Zhou, Y. and Voyiadjis, G.Z., 2019. Finite element modeling of production-induced compaction and subsidence in a reservoir along coastal Louisiana. Journal of Coastal Research, 35(3), 600–...

Rock Physical Controls on Production-induced Compaction in the Groningen Field

Advancing production from the Groningen gas field to full depletion generates substantial, field-scale deformation, and surface subsidence. Quantifying associated risk requires understanding physical processes in the subsurface, in particular those related to deformation of the Permian sandstone reservoir. Here, we report the results of a large experimental study, using fresh core material taken from the center of the field. By subjecting the material to depletion and slight unloading, complemented with a range of rock property measurements, we determine what rock physical properties control production-induced compaction in the material. Our results show that, although a large part of the deformation can be explained by classical linear poroelasticity, the contribution of inelastic (permanent) deformation is also significant. In fact, it increases with progressing pressure depletion, i.e. with increasing production. Utilizing univariate and multivariate statistical methods, we explain the additional inelastic deformation by direct effects of porosity, packing, and mineral composition. These proxies are in turn related to the depositional setting of the Permian reservoir. Our findings suggest that field-scale subsidence may not only be related to the often-used rock porosity, but also to packing, and composition, hence the local depositional environment. This motivates alternative assessments of human-induced mechanical effects in sedimentary systems.

Mechanical behaviour of the Rotokawa Andesites (New Zealand): Insight into permeability evolution and stress-induced behaviour in an actively utilised geothermal reservoir

High-enthalpy geothermal systems, such as that of Rotokawa in the central Taupo Volcanic Zone (TVZ) of New Zealand, commonly host fracture systems that control reservoir permeability. The loss of porosity and permeability through compaction and mineral precipitation, and the opening of permeable fracture zones, occurred in tandem as the geothermal system evolved. In order to better understand how porosity can evolve in an active reservoir, a systematic study of triaxial deformation experiments has been undertaken to investigate the mechanical behaviour and failure mode of coherent andesite lavas and andesite breccias (the predominant reservoir lithologies) with varying porosity and alteration intensity from the Rotokawa Geothermal Field under high confining pressure and high pore fluid pressure conditions. These experiments provide insight into the formation of permeable fracture zones and low-permeability zones within the reservoir. Although we observed brittle failure mode in all our experiments (covering a range of rock attributes and relevant pressure conditions), we find that the amount of dilatancy during deformation, inferred to assist in the creation of a permeable network, is considerably reduced at high effective pressures and for high-porosity and highly altered samples. Importantly, some high-porosity and/or highly altered samples experienced a net compaction at the end of the experiments. Microstructural observations suggest that, although the failure mode remains ultimately brittle, microcracking and pore collapse can operate in concert at high effective pressure, porosity, and/or alteration, potentially explaining the reduction in dilatancy. We infer, therefore, that the deformation of the rocks within the reservoir could result in either a bulk permeability increase or a permeability decrease, depending on the physical characteristics of the rocks (e.g. porosity and alteration intensity) and the pressure conditions. Using our new experimental data we provide a Hoek-Brown failure criterion for an intact Rotokawa andesite with a representative porosity and show the distribution of stresses around a vertical and deviated borehole during drilling using a finite element model. Finite element modelling shows that brittle failure of intact rock (and therefore off-fault seismicity) during production and injection is unlikely in RKA containing the average porosity assumed for the reservoir. Production-induced compaction and injection-induced tensile failure could occur however in high-porosity and/or highly altered intact rock (which could lead to instability at the well boundary), particularly in deviated wells. The experiments and modelling presented herein provide insight into the evolution of an actively utilised geothermal reservoir.

Monitoring Primary-Depletion Reservoirs With Seismic Amplitudes and Time Shifts

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18219, "Monitoring Primary-Depletion Reservoirs With Seismic Amplitudes and Time Shifts," by A. Tura, T. Barker, P. Cattermole, C. Collins, J. Davis, P. Hatchell, K. Koster, P. Schutjens, SPE, and P. Wills, Shell Intl. E&P, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. In the high-porosity, poorly consolidated turbidites of the deepwater Gulf of Mexico (GOM), production-induced compaction can be the drive mechanism when aquifer support is weak and before pressure support by secondary-recovery water injection begins. Time-lapse (4D) seismic-monitoring time shifts occur in areas of depletion and in the overburden, and they indicate compartmentalization in the reservoir. Compartmentalization information can help place new production and injection wells better, as well as new sidetracks for optimized field development. Introduction Travel-time changes are more sensitive to reservoir compaction caused by depletion than to fluid-saturation changes. As a result, travel-time changes are simpler to interpret compared to amplitude changes. In primary-depletion reservoirs, the 4D amplitude and travel-time changes are not large. Small-to-moderate amplitude changes manifest at the oil/water contact (OWC) or in large pressure-depletion areas. The travel-time changes can be on the order of one to several time samples of the typical seismic-sampling interval of 4 milliseconds. In the GOM, the overburden can contain large faults, salt bodies, steep dips, near-surface channels and bodies, diffractors, and overpressured regions. As a result, 4D monitoring of these reservoirs requires highly repeatable seismic data with very accurate positioning of sources and receivers between surveys. In the GOM, it is often difficult to obtain high repeatability with 3D streamer surveys because of strong currents. A novel two-boat, three-pass acquisition method to overcome acquisition-repeatability issues was investigated and used by the Shell Mars/Europa 4D study team over the Mars and Europa fields. It was possible to match amplitude changes computed from synthetic seismic (by use of history-matched reservoir-simulation models) as well as travel-time changes computed from geomechanical models of these fields. Mars and Europa Fields Field data from Mars and Europa fields demonstrate the concepts detailed in the full-length paper. Mars is a large field in approximately 4,000-ft water depths with approximately 500 million bbl of recoverable oil remaining from 18 vertically stacked producing reservoirs. Most production is from four main reservoirs, with up to 7,000 psi pressure depletion and with the deepest reservoir being the largest. Waterflooding began in 2004 to recover remaining reserves. Time-lapse surveys of this field are important to monitor the waterflood and detect compartmentalization. Europa is a subsea-completed field and is connected to the Mars platform. It has two producing reservoirs, with up to 4,000 psi pressure depletion, and has been undergoing primary depletion since January 2000. Well performance indicated that both producing reservoirs are compartmentalized by small-scale faults, and production problems have been encountered including well failures and fluid-flow impairment. Time-lapse seismic was used in this field to detect compartmentalization and movement of the OWC.

Monitoring primary depletion reservoirs using amplitudes and time shifts from high-repeat seismic surveys

In the high-porosity, poorly consolidated turbidites of the deepwater Gulf of Mexico, production-induced compaction is the main production-drive mechanism when aquifer support is weak and prior to pressure support by secondary recovery water injection. Time-lapse (4D) seismic monitoring of this class of reservoirs has provided several new learning opportunities. The time-lapse amplitude response of these fields can be complicated due to saturation changes (water replacing oil) inside the reservoir, rock compaction causing density and velocity changes inside the reservoir, stress relief and associated deformation of the rock outside the reservoir, and changes in reservoir fluid pressures due to pore-pressure decrease. Methods that rely on time-lapse amplitude changes with offset to discriminate pressure and saturation changes can help separate and thus simplify the interpretation of some of these effects (Tura and Lumley, 1999; Landro, 2001).

Production-Induced Compaction of a Sandstone Reservoir: The Strong Influence of Stress Path

Summary The decrease of pore pressure during hydrocarbon production (depletion) leads to compaction of the reservoir, which in turn changes the stresses acting on the reservoir. The prediction of reservoir compaction and its consequences is usually based on laboratory experiments performed under uniaxial strain conditions, i.e., allowing no lateral strain during depletion. Field data of the Groningen gas field (The Netherlands) indicate that the stress development of the field deviates significantly from the stress path under uniaxial strain conditions. Laboratory experiments show that the applied stress path has a strong influence on the depletion-induced compaction behavior. We discuss the consequences of these results for the field compaction behavior by considering the responsible deformation mechanisms active in reservoir and experiment. The new Groningen field data, in combination with our experimental results, provide an explanation for the difference between the prediction of compaction and subsidence based on uniaxial experiments and the measurement of compaction and subsidence in the Groningen field. With the use of the new stress path, the predicted and measured compaction and subsidence are in agreement.

Production-Induced Compaction of the Brent Field: An Experimental Approach

Summary The most attractive option to increase hydrocarbon recovery from the Brent and Statfjord reservoirs (Brent field, North Sea) is a gradual decrease of the reservoir fluid pressure. To provide laboratory data upon which estimates of the resulting reservoir compaction and surface subsidence could be made, we performed uniaxial compaction experiments at room temperature and in-situ stress conditions. Experimental observations suggest that the reloading of core samples to in-situ stress conditions closed coring-induced microcracks. Most Brent samples (porosity 6% to 30%) showed a compressibility that was constant with increasing effective stress (i.e., linear compaction) and increased with porosity. There was 30% to 75% strain recovery during unloading and an average permanent shortening of about 0.7%. Some high-porosity (25% to 30%) Brent samples showed a relatively high uniaxial compressibility that increased with stress (nonlinear compaction). Nearly all the Statfjord samples showed nonlinear compaction. The Statfjord samples from Core A, with a porosity of 8% and 20% to 22%, showed about 45% strain recovery upon unloading and an average permanent shortening of 0.6%. The Statfjord samples from Core B (porosity 22% to 28%) showed only 24% strain recovery and an average permanent shortening of 3.2%. Microscopy analysis of the Brent samples revealed no evidence for a compaction-induced change in microstructure. In contrast, the Statfjord samples from Core B (22% to 28% porosity) displayed numerous intergranular and transgranular cracks in quartz and feldspar grains, which probably triggered grain sliding and grain rotation. These inelastic brittle mechanisms, which occur together with elastic deformation of the load-bearing grain framework, seem dominant in high-porosity (Statfjord-type) sandstones and should be incorporated into predictive models of production-induced compaction of quartz-rich reservoir rock.