Sau-Wai Wong

HYDRATION SWELLING OF WATER‐ABSORBING ROCKS: A CONSTITUTIVE MODEL

Water-absorbing rocks are formed from minerals that can hold water in their crystal structure or between grain boundaries. Such water absorption is often accompanied by a change in the crystal dimension that manifests itself as a swelling of the rock. Swelling is particularly pronounced in rocks containing phyllosilicates because of the ease with which these minerals hydrate; it is thus of geological and geotechnical relevance in shales, clay-rich soils and zeolitized tuffs. The model of hydration swelling that we present here is based on extended versions of the equations of poroelasticity and Darcys transport law, which we derive using a non-equilibrium thermodynamics approach. Our equations account for the hydration reaction under the assumption that the reaction rate is fast in comparison with the rate at which hydraulic state changes are communicated through the rock, i.e. that local physico-chemical equilibrium persists. Using a finite-element scheme for solving numerically the governing equations of our model, we simulate the creep of shales during a routine swelling test and calculate the stress and strain distributions around wellbores drilled in shale formations that undergo swelling. We show that swelling effects promote tensile failure of the wellbore wall.

Borehole Stability in Shales

Downhole mud/shale interaction can only be properly understood if rock mechanical, shale hydration, and fluid transport phenomena are taken into account. This paper presents a review of Koninklijke Shell E P Laboratoriums research on borehole stability in shales. Mechanisms relevant to shale stability, including pore pressure penetration (the gradual increase in pore pressure resulting from high mud weight), capillary threshold pressures, compressive and tensile failure, postfailure stabilization, hydration stress, inhibition, and osmotic phenomena are discussed. The authors attempt to integrate these mechanisms into a comprehensive model for shale behavior.

Interaction of Multiple Hydraulic Fractures in Horizontal Wells

The technology of multiple hydraulic fracture stimulation in horizontal wells has trans‐ formed the business of oil and gas exploitation from extremely tight, unconventional hydro‐ carbon bearing rock formations. The fracture stimulation process typically involves placing multiple fractures stage-by-stage along the horizontal well using diverse well completion technologies. The effective design of such massive fracture stimulation requires an under‐ standing of how multiple hydraulic fractures would grow and interact with each other in heterogeneous formations. This is especially challenging as the interaction of these fractures are subject to the dynamic process of subsurface geomechanical stress changes induced by the fracture treatment itself. This paper consists of two parts. Firstly, an idealised analytical model is used to highlight some key features of multiple hydraulic fractures interaction, and to provide a quantifica‐ tion of ‘stress shadow’. Secondly, a new non-planar three dimensional (3D) hydraulic frac‐ turing numerical model is used to provide an insight into the growth of multiple fractures under the influence of subsurface geomechanical stress shadows. Attention is given to studying the height growth of multiple fractures.

The Geomechanical Interaction of Multiple Hydraulic Fractures in Horizontal Wells

The technology of multiple hydraulic fracture stimulation in horizontal wells has trans‐ formed the business of oil and gas exploitation from extremely tight, unconventional hydro‐ carbon bearing rock formations. The fracture stimulation process typically involves placing multiple fractures stage-by-stage along the horizontal well using diverse well completion technologies. The effective design of such massive fracture stimulation requires an under‐ standing of how multiple hydraulic fractures would grow and interact with each other in heterogeneous formations. This is especially challenging as the interaction of these fractures are subject to the dynamic process of subsurface geomechanical stress changes induced by the fracture treatment itself. This paper consists of two parts. Firstly, an idealised analytical model is used to highlight some key features of multiple hydraulic fractures interaction, and to provide a quantifica‐ tion of ‘stress shadow’. Secondly, a new non-planar three dimensional (3D) hydraulic frac‐ turing numerical model is used to provide an insight into the growth of multiple fractures under the influence of subsurface geomechanical stress shadows. Attention is given to studying the height growth of multiple fractures.

Characterizing Natural-Fracture Permeability From Mud-Loss Data

Original SPE manuscript received for review 20 October 2009. Revised manuscript received for review 29 March 2010. Paper (SPE 139592) peer approved 25 May 2010. Summary Liétard et al. (1999, 2002) have provided important insight into the mechanism and prediction of transient-state radial mud invasion in the near-wellbore region. They provided type curves describing mud-loss volume vs. time that allow the hydraulic width of natural fractures to be estimated through a curve-matching technique. This paper describes a simpler and more direct method for estimating the hydraulic width by the solution of a cubic equation, with input parameters given by the well radius rw, the overpressure ratio p/ y, and the maximum mud loss volume (Vm)max.

The Rock Mechanical Aspects of Drilling a North Sea Horizontal Well

In view of the high cost of extended reach horizontal drilling, a borehole stability study was conducted to determine the range of mud weights suitable for drilling the first horizontal well in a North Sea oil field. The stability of the borehole was assessed using three methods that differ in complexity, cost, and conservatism. The study shows that drilling the horizontal section is feasible from a rock-mechanical viewpoint if a mud weight slightly above the pore pressure is used.

Pulsed Water Injection During Waterflooding

During a half-year field test a novel method was applied for water injection during waterflooding in a weakly consolidated, heavy oil reservoir (90-120 cP). The injection has been combined with a hydraulic pulsing tool downhole in the injection well to provide additional dynamic pressure pulses on the order of 4-17 bar, with 5-6 pulses per minute. The technology, developed in Canada and applied successfully, especially as a well stimulation technique in order to initiate or stimulate oil production with sand coproduction. The first objective was to see whether pulsing would be beneficial for the efficiency of the injection process. Furthermore, laboratory experiments and theoretical developments suggest that pulsing might improve the sweep efficiency of the flooding pattern. Hence, the promise of this technique would be potentially faster and higher oil recovery during waterflooding. In the design of the field test it was chosen to keep the total water injection rate on the same level as before pulsing was applied, on the order of 110 m3/d. The rationale behind this decision was that previous experience in the field has shown that higher injection rates resulted in pressurization of the reservoir and increased fingering. In addition, the fixed injection rate allowed us to focus on improvements in sweep efficiency, without correcting production figures for the higher injection rate. Pressure Pulse Technology (PPT) was applied without any significant operational problems for half a year although severe corrosion problems unrelated to the PPT project were uncovered after the trial. Injection and production performance has been monitored before, during and after the test. When pulsing started, injection pressure dropped, and even after the pulsing stopped a lower wellhead pressure has been measured. With constant injection rate this shows an improvement in injectivity. It also indicates a significant reduction in near wellbore skin factor or possible improved injection conformance. The injection water used is considered dirty and potentially deteriorates injectivity over the life of the well. Indications are that injection pressure is now slowly building up again following the trial. Improvements in production have not been confirmed by this field trial. The accuracy and repeatability of the production measurements have not assisted in identifying the potential effect. However, the field trial results have enabled us to recognize the potential use of the technology for an efficient high -rate injection strategy, which would avoid injection under fracturing conditions. We outline situations where such applications would be desirable. Moreover, we believe the pulsing technique can be applied for efficient waste disposal. Higher injection rate and more efficient pulsed injection potentially lead to improved recovery, although actual improvements need to be assessed with further tests. Introduction During waterflooding, water is injected in order to sweep the remaining oil towards the producers. The volumetric sweep efficiency is influenced by factors such as the mobility ratio, gravitational and capillary forces, injection rate, and reservoir heterogeneity. Higher injection rate promotes faster recovery and supress gravity dominated water underruns. High pressure resulting from high injection rates can induce fractures, which can cause early water breakthrough by creating a preferential flow path to the producing wells. Higher pressures can also lead to accelerated viscous instabilit ies leading to poor sweep efficiency. Slower injection promotes capillary crossflow beneficial for increasing sweep in lower permeability layers. Summarizing, the optimum injection/production strategy will depend on the particular reservoir. Many technologies aim at improving the recovery of a flooding pattern, either by changing the physical properties of the injected fluid or by changing the injection and production strategy. A relatively inexpensive technology that has been reported2 to lead to improved recovery is the use of non-steady state waterflooding, or cyclic waterflooding based on using changes in injection rates over periods of days to months. Laboratory research in Canada investigated the use of rapid pressure pulses on the flooding of core samples. In this case the water injection is combined with short period (order 1-5 SPE Paper 84856 Pulsed Water Injection during Waterflooding Jeroen Groenenboom, SPE; Sau-Wai Wong, SPE, Shell Intern. Expl. & Prod.; Tor Meling, SPE; Robert Zschuppe; Brett Davidson, Prism Production Technologies Inc.

The Use of Multi-lateral Well Technology in an Infill Development Project

Multi-lateral well technology has been employed in a recent infill development project in the South Furious field, offshore the state of Sabah in Malaysia. The main motivation for considering multi-lateral well technology is to increase the drainage area, i.e., to connect more reservoir compartments, for a fixed number of available well slots. The successful application of this emerging technology opens up a whole new approach of further developing oil reserves in South Furious and other fields. The multi-lateral well with a cased and cemented junction is the first in Malaysia and perhaps, the Asia-Pacific Region. It was successfully drilled, completed and opened for production. In this paper, the logic and philosophy in employing this technology for South Furious infill development are presented. We highlight the key design concerns, risks and costs. The paper describes only the essential aspects of the drilling and completion operations. We discuss the lessons learnt, the production experience to-date, and offer some recommendations for future implementation.

New Model of Permeability of Tight Shale Gas Rock Based on Open-System Geomechanics

Finding an adequate description of permeability in shale gas is a challenging problem because of complexity of the rock which contains both organic and non-organic components. Due to small pore size, which is comparable with the mean free path of molecular motion, matrix permeability should include Knudsen and slip flow components as well as conventional Poiseuille one. If we assume as usual that permeability is a function of porosity, k(), then stress and pore pressure dependence of variation of permeability can be found as determined by porosity related effective (Terzaghi) stress, i.e. difference between confining stress and pore pressure (Carroll, 1980; Charlez, 1997):