Sally Ann Hamilton

Predicting Rock Mechanical Properties from Wireline Porosities

The rock mechanical behaviour of reservoir rocks is important in the design and implementation of drilling and production programmes. Traditionally rock mechanical properties are obtained from direct measurement on core samples or from mechanical calculations on acoustic wireline log measurements. This paper reports the rock mechanical properties of many different reservoir rocks of different porosities. This has led to the development of a new method of predicting rock mechanical properties directly from porosity. The paper discusses the measurement of experimentally derived porosity, elastic moduli and fracture strength parameters and the interpretation of these mechanical properties results into direct correlations with porosity. The application of these results to obtain continuous rock mechanical property plots of the reservoir from wireline derived porosity is discussed. The practical use of these rock mechanical property profiles in drilling, production and enhanced reservoir simulation is also emphasised. Porosity (Φ), modulus of elasticity (E), Poissons Ratio (ν), uniaxial compressive strength (UCS), cohesion (τ 0 ), angle of internal friction (ψ), and triaxial stress factor (k), were measured on samples from a wide range of North Sea reservoirs using a conventional triaxial testing machine. This paper describes the procedure used and presents the correlations obtained from plotting each of the rock mechanical properties against porosity. The derivation of wireline porosities along with empirical corrections are presented and the results of applying the correlations to these wireline derived porosities to produce continuous rock mechanical property plots are discussed. Logs were calibrated to core-measured values to reveal realistic elastic and inelastic moduli profiles. The continuous property logs provide a reasonable estimate of the possible behaviour at discrete points throughout the reservoir interval, but they are limited in their description of the behaviour of individual beds as coherent bodies. A technique has been developed to pick out these individual beds and assess how they will perform as Rock Mechanical Coherent Units, i.e. sets of beds that perform in a similar or dissimilar manner to adjacent layers. Finally a discussion on how the results are used to aid production and generate enhanced reservoir simulation will be presented.

Permeability prediction using stress sensitive petrophysical properties

The correlation of stress sensitivity to various petrophysical parameters was studied by analysis of experimental results from a range of sandstone core plugs tested hydrostatically at room temperature. The parameters measured were: compressional wave velocity, porosity, permeability and electrical resistivity. More detailed information on the effects of sorting and grain size distributions was obtained from experiments on artificial, unconsolidated sandstone cores. The measurements showed a high degree of stress sensitivity, which was different for each core but, broadly, could be classified as either high or low stress sensitivity. Cores from the high permeability clean sand were less stress sensitive than the cores from the low permeability coarsening-upwards sequence and the petrophysical values when combined into a synthetic log distinguished between the two lithologies. The results were compared to the predictions of a simple asperity deformation model. The experimental results and the model suggested a possible logging strategy to deduce permeability, by varying wellbore pressure.

Geomechanical modelling of CO2 geological storage with the use of site specific rock mechanics laboratory data

Many diverse challenges – political, economic, legal and technical – face the continued development and deployment of geological storage of anthropogenic CO2. Among the technical challenges will be the satisfactory proof of storage site security and efficacy. Evidence from many past geotechnical projects has shown the investigations and analyses that are required to demonstrate safe and satisfactory performance will be site specific. This will hold for the geomechanical assessment of saline aquifer storage site integrity where, compared to depleted hydrocarbon fields, there will be no previous pressure response history or rock property characterization data available. The work presented was carried out as part of a project investigating the improvement in levels of confidence in all aspects of saline aquifer site selection and characterization that could be expected with increasing data availability and in-depth analysis. Attention focused on the geomechanical modelling and the rock mechanics data used to populate models of two storage sites in geological settings analogous to those where CO2 storage might be considered. Coupled geomechanical models were developed from reservoir simulation models initially incorporating generic rock mechanical properties and then laboratory-derived site-specific properties. The models were run in various configurations to investigate the effect of changing the rock mechanical properties on the geomechanical response of the storage systems. Modelling results showed that the pressure response at one site due to low injectivity caused significant potential for fault reactivation. Increasing the number of injection wells, thereby reducing the individual rates needed to deliver the target capacity, reduced the injection pressures and ameliorated, but did not eliminate, this adverse response.

Correlation Between Microstructure and Flow Behavior in Porous Sandstones

Abstract The correlation between permeability and petrographical parameters in cored samples can be used by extrapolations to predict permeability in uncored intervals. The core analysis described here is concerned with the study of fluid-rock interactions in rock samples from three sandstone reservoirs, in particular the effect of petrographical parameters on flow behavior. A positive correlation between liquid permeability and the volume fraction of silica is clearly demonstrated. Liquid permeability was correlated using multivariate regressions to one to five petrographical parameters, the results of which have useful application in the estimation of reservoir permeability where samples are not available for experimental testing.

Velocity and attenuation measurements during 2 phase core flooding

Reservoir saturation changes can be detected by repeat 3-D seismic surveys. These rely on some seismic attribute such as travel time, amplitude or velocity changing with saturation, either gas, oil or water. Rock physics measurements show that rocks with a high compliance or low stiffness have the greatest sensitivity of Vp to saturation changes. Stiff rocks may only exhibit small changes in Vp with saturation, which may be undetectable on repeat surveys. However other seismic attributes, such as signal amplitude may show large variations.

Measurement of Velocity Changes Induced In Cores By Saturation Changes, Water Flood, and Drawdown.

4 D seismic measurements are used to monitor changes during the production and stimulation of a reservoir. Parameters such as the type of fluid fill, porosity and pressure distribution are obtained from the results. Interpretation is difficult and is complicated by stress sensitive matrix properties, and phase sensitive fluid properties. Interpretation depends on some theoretical scheme such as Gassmann’s equation. Such formulations require a detailed knowledge of the velocity of sound in the fluids present and compression and shear wave velocities in the n~atrix. These velocity values are obtained from a combination of field and laboratory measurements. Introduction 4D or time lapse seismic surveying permits the possibility of monitoring the spatial distribution of fluids in oil and gas reservoirs 1. A knowledge of the spatial distribution of fluids in a reservoir allows for more efficient exploitation strategies such as infill drilIing’. 4D seismic uses two combined 3D seismic surveys separated in time. 4D seismic surveying has been used to track drainage ] and pressure compartmentalization ~,~C02 flooding using time lapse cross borehole tomography and various other types of stimulation. The two 3D surveys are analyzed to show differences in acoustic propertiessuch as impedance. The differences in acoustic properties are interpreted in terms of production induced changes in pressure, temperature and fluid saturation 5. Implicit in the analysis is a knowledge of the way in which the acoustic properties of the rock fluid system respond to changes in pressure, temperature and saturation. There have been many petroacoustic studies of the rock fluid system 6. These studies tend to show changes in acoustic properties with pressure and saturation. Such changes in properties, could be detected with 4D seismic surveys, assuming the results scale with frequency. The petroacoustic studies are performed at high frequencies, of tbe order of lMHz 6, seismic surveys tend to be less than 100Hz’ and borehole seismic around 1kHz 4. Test Equipment The test equipment comprises a servo-controlled stiff testing machine, a Hock cell, pressure intensifier (for confining pressure), seismic velocity system, pore fluid system and data logging system. Servo-controlled Stiff Testing Machine and Pressure Intensifier The loads applied to the samples were generated using a servo-controlled hydraulic stiff testing machine rated to 1000kN This machine consisted of a straining frame with a hydraulic ram, several platens and a load cell. The ram operates veflically with the load cell located in the cross head of the straining frame. The position of the ram is monitored by a electronic LVDT and this position signal together with that from the load cell allows tbe flow of hydraulic oil to the ram to be controlled. Thus the load and rate of loading are controlled. In the current set of experiments the testing machine was only used to supply an axial load to the sample to give an overall isotropic stress system. Associated with tbe stiff testing machine is a pressure intensifier which uses the a similar servo-hydraulic circuit to control the confining pressure in the Hock cell. Hock Cell This ceil provided a means of applying radial pressure to the sample (Fig. 1). It consisted of a steel ceil rated to 70MPa within which is located a polyurethane sleeve. Hydraulic pressure is transmitted to the radial surface of the sample via oil retained within the annulus between the body of the cell and the sleeve. The samples were located within the sleeve and the hydraulic oil pressurized by the a servocontrolled pressure intensifier. The pressure and volume of oil introduced or removed from the cell were monitored electronically during the tests. The tests were conducted at ambient temperature.