Keith Rawnsley

Joint spacing: analogue and numerical simulations

Abstract Joint spacing distribution laws remain a controversial subject in the literature. In order to understand joint spacing, analogue and numerical modelling techniques were used. In the analogue model an evolution of the joint spacing distribution law during fracture development was observed as new fractures appear. Different processes for fracture initiation and propagation have been tested numerically including mid-point fracturing and partially or completely random processes. This study suggests that an initiation of fractures according to a random process could explain real joint spacing distributions. An evolution of joint spacing distribution laws from initially negative exponential to log-normal and normal is found with increasing joint development.

Slip distributions on intersecting normal faults

Abstract Slip distributions on normal faults often are asymmetric and display multiple local maxima. Numerically computed slip distributions from elastic models indicate that such irregular slip distributions can be caused by mechanical interaction between intersecting faults that produce local perturbations of the stress field resolved on the faults. A three-dimensional boundary element model based on the displacement discontinuity method has been used to analyze the mechanical interaction of faults that form Y- or T-shaped intersections. Slip distributions are asymmetric with a steeper slip gradient toward the line of intersection. Multiple slip maxima occur, depending on the angle between the faults, but generally they are not located along the intersections, nor at the fault centers. Examples of intersecting normal faults taken from oil reservoir seismic surveys and sandbox experiments have been used to confirm the application of the theoretical results. Despite considerable simplifications in terms of geometry and boundary conditions, the computed slip distributions capture the characteristics of the observed slip distributions.

Joints in the Mesozoic sediments around the Bristol Channel Basin

Abstract Analysis has been carried out at four locations on the edges of the Bristol Channel Basin to illustrate the later phases of deformation of a sedimentary basin, and to illustrate the control on joint patterns of subtle changes in the stress system. The characteristics of the joints are described and influences on joints are determined, including the roles of faults, folds and beds. There is a low coefficient of correlation between joint spacing and bed thickness, except in very thin limestone beds, which have a high density of joints. The lengths and spacings of earlier joint phases are usually greater than those of later phases. Later joints normally abut against earlier joints. The joints abut the latest faults but are not displaced by them, so the joints post-date the main Alpine contraction. The joints formed in five main phases during reduction of the Alpine stresses. Phase 1 joints are sub-parallel to the regional compression direction (160–180°). Phase 2 joints are perturbed by faults, often curving towards points of stress concentrations along the faults. Phase 3 joints are sub-parallel to the earlier E–W-striking fold axes. Phase 4 joints are cross-joints, and phase 5 joints form polygonal patterns within joint-bound blocks. Phases 2 and 3 do not occur in the absence of faults and folds, and correspond with a reduction in horizontal compression and an increase in the importance of local factors. Phases 4 and 5 occur at all locations.

Discriminating Fracture Patterns in Fractured Reservoirs by Pressure Transient Tests

A new approach is described to characterise fractured reservoirs using a software code which can simulate transient well tests in a realistic 3D fracture/matrix model. By a process of iteration the geological and flow data can be reconciled to produce plausible models which could help to constrain reservoir simulation. The numerical results indicate that the simulated pressure derivatives generally show different characteristics with different fracture pattern configurations. The results also indicate that theoretical dual porosity behaviour is absent when a more realistic transient matrix to fracture flow solution is used. This approach has important implications in the use of well tests to determine parameters for input to a multiphase flow simulator, where the well positioning in relation to highly conductive fractures is critical. The advantage of the described approach, compared to conventional analytical analysis, is that the geological data are calibrated and preserved. Parameters can then be more accurately represented in a dynamic reservoir model.

Extrapolating fracture orientations from horizontal wells using stress trajectory models

This paper presents a recently developed approach for extrapolating fracture orientations from horizontal wells. It is commonly accepted that tension fractures propagate perpendicular to the minimum principal stress. The hypothesis has been tested that in some cases, by modelling the stress trajectories within a faulted compartment, it is possible to reproduce the orientations of the tension fractures. The stress trajectory models we present are based on photo-elastic techniques. The models are first calibrated against outcrop fracture patterns and are subsequently applied to a reservoir database consisting of a 3D seismic block and a horizontal well with almost complete FMS image coverage. The results show that tension fractures may show perturbed orientations in the vicinity of faults. It is shown that by appropriate characterization of the seismic and well fracture data, it may be possible to use stress trajectory models to predict fracture orientation in a reservoir.

Development of joint sets in the vicinity of faults

Unlike regional joint patterns, fault-related joints can often be related to stress perturbations. Their patterns, which have seldom been described, are sometimes complex and difficult to decipher. The examples presented here are mainly joints associated with meter to decameter scale pre-existing faults in limestone. The joints mostly tend to be parallel or perpendicular to the faults with the occurrence of sets of diverging joints localized along the faults. The patterns presented are interpreted in the light of photoelastic models which enable the prediction of stress deviations linked to subtle morphological details on faults. These models show that many of the observed features can be interpreted as stress deviations linked to alternation of closed and open segments along the faults. Joints appear as excellent markers of the deviation of the major horizontal principal paleostress trajectories. A mechanical discussion based on the paradox of the absence of concentration of joints in zones of tensile stress concentration leads to the idea that joint propagation is assisted by contraction within the layers.