J.W. Dudley

The Mounds Drill-Cuttings Injection Field Experiment: Final Results and Conclusions

This paper summarizes the results obtained from a comprehensive, joint-industry field experiment designed to improve the understanding of the mechanics and modeling of the processes involved in the downhole injection of drill cuttings. The project was executed in three phases: drilling of an injection well and two observation wells (Phase 1); conducting more than 20 intermittent cuttings-slurry injections into each of two disposal formations while imaging the created fractures with surface and downhole tiltmeters and downhole accelerometers (Phase 2); and verifying the imaged fracture geometry with comprehensive deviated-well (4) coring and logging programs through the hydraulically fractured intervals (Phase 3). Drill cuttings disposal by downhole injection is an economic and environmentally friendly solution for oil and gas operations under zero-discharge requirements. Disposal injections have been applied in several areas around the world and at significant depths where they will not interfere with surface and subsurface potable water sources. The critical issue associated with this technology is the assurance that the cuttings are permanently and safely isolated in a cost-effective manner. The paper presents results that show that intermittent injections (allowing the fracture to close between injections) create multiple fractures within a disposal domain of limited extent. The paper also includes the conclusions of the project and an operational approach to promote the creation of a cuttings disposal domain. The approach introduces fundamental changes in the design of disposal injections, which until recently was based upon the design assumption that a large, single storage fracture was created by cuttings injections.

Broadband acoustic emission observations during fracture propagation in rock-like material

Abstract Acoustic emission (AE) is used for monitoring fracture growth in many materials, including concrete and rock. Using a robust location algorithm allows event location from first arrival times to be determined by conventional commercial resonant sensors; however, commercial sensors modify the AE signals to a such a degree that quantitative analysis is all but precluded. Quantitative waveform analysis is required if microseismic signals are to be used to identify event magnitudes and specific fracture mechanisms. As part of a joint research effort between Shell, UIC, and UC on fracture mechanisms in rocks, we have developed a high-fidelity acoustic emission sensor specifically designed for use with rock. The sensors can be surface mounted or embedded directly into the material. This paper reports acoustic emission observations made with an array of these new high-fidelity sensors. AE signals were recorded during loading (fracture propagation) and unloading on plate grout specimens subject to splitting tests. AE events during loading were seen to originate primarily in a limited volume ahead of the crack tip, while unloading events generally came from the macroscopic crack. From first motion analysis, typical loading events appeared to be tensile in nature, while unloading events often had shear components. The median frequency and bandwidth of the loading events was seen to be greater than that of unloading events.

Stress And Pore-pressure Dependence of Sound Velocities In Shales: Poroelastic Effects In Time- Lapse Seismic

We have measured the stress, stress-path, and pore-pressure dependence of p-wave velocities of different shales. In good approximation, for small deformations, vertical velocity changes are proportional to changes in vertical effective stress. For low-permeability, undrained formations such as shales, it has been well established that pore pressures change as a function of mean-total stress changes and deviatoric stress changes. Analytical and finite-element geomechanical modeling, for a linear-elastic, isotropic half space, demonstrate that a ubiquitous porepressure increase in the low-permeability non-producing formations accompanies depletion of an adjacent reservoir. This pore pressure increase results in positive seismic timelapse time shifts in the overand underburden and may nearly cancel the negative time shifts due to archinginduced total vertical stress increase in the sideburden. Thus, poroelastic effects might offer an alternative explanation for the observation of mostly positive time shifts in time-lapse seismic, which was previously attributed to an asymmetric velocity-strain dependence for loading and unloading.

ISRM Suggested Method for Uniaxial-Strain Compressibility Testing for Reservoir Geomechanics

Please send any written comments on this ISRM Suggested Method to Prof. Resat Ulusay, President of the ISRM Commission on Testing Methods, Hacettepe University, Department of Geological Engineering, 06800 Beytepe, Ankara, Turkey at resat@hacettepe.edu.tr .

Statistical Aspects of Microheterogeneous Rock Fracture: Observations and Modeling

Rocks and other geomaterials are heterogeneous materials, with a well-recognized hierarchy of defects from micro-heterogeneities on the grain level to a large-scale network of cracks and layering structures. Their nature create a challenge for determining macroscopic properties, particularly for properties that are scale dependent, complicating both the property measurement and its appropriate application in modeling. This paper discusses the concept of a “representative volume”, which is commonly used in modeling microheterogeneous but statistically homogeneous material by an effective homogeneous continuum. The foundation of this concept is presented, along with its limitations in dealing with properties like strength and fracture toughness that exhibit a scale effect. This limitation is illustrated with a study of brittle fracture of a concrete where it is considered a model for statistically homogeneous rock. The study includes determining a scaling rule for the scale effect in fracture toughness, and shows that the fracture of brittle materials like rocks and concrete appears in the form of highly tortuous, stochastic paths. This reflects a complex interaction between a crack and pre-existing as well as newly formed micro-defects controlled by chance, and results in a large scatter of all fracture-related parameters. This behavior suggests a synthesis of fracture mechanics with probability and statistics, and so a brief exposition of statistical fracture mechanics (SFM) that addresses the statistical aspects of fracture is also presented. SFM is a formalism that combines fracture mechanics methods with probability theory and serves as the basis for an adequate modeling of brittle fracture.

Active and Passive Acoustic Imaging Inside a Large-Scale Polyaxial Hydraulic Fracture Test

An automated laboratory hydraulic fracture experiment has been assembled to determine what rock and treatment parameters are crucial to improving the efficiency and effectiveness of field hydraulic fractures. To this end a large (460 mm cubic sample) polyaxial cell, with servo-controlled X,Y,Z, pore pressure, crack-mouth-opening-displacement, and bottom hole pressure, was built. Active imaging with embedded seismic diffraction arrays images the geometry of the fracture. Preliminary tests indicate fracture extent can be imaged to within 5%. Unique embeddible high-fidelity particle velocity AE sensors were designed and calibrated to allow determination of fracture source kinematics.