William Rizer

Fracture detection using crosswell and single well surveys

We recorded high‐resolution (1 to 10 kHz), crosswell and single well seismic data in a shallow (15 to 35 m), water‐saturated, fractured limestone sequence at Conocos borehole test facility near Newkirk, Oklahoma. Our objective was to develop seismic methodologies for imaging gas‐filled fractures in naturally fractured gas reservoirs. The crosswell (1/4 m receiver spacing, 50 to 100 m well separation) surveys used a piezoelectric source and hydrophones before, during, and after an air injection that we designed to displace water from a fracture zone. Our intent was to increase the visibility of the fracture zone to seismic imaging and to confirm previous hydrologic data that indicated a preferred pathway. For the single well seismic imaging (a piezoelectric source and an eight‐element hydrophone array at 1/4 m spacing), we also recorded data before and after the air injection. The crosswell results indicate that the air did follow a preferred pathway that was predicted by hydrologic modeling. In addition,...

Inverse hydrologic modeling using stochastic growth algorithms

We present a method for inverse modeling in hydrology that incorporates a physical understanding of the geological processes that form a hydrologic system. The method is based on constructing a stochastic model that is approximately representative of these geologic processes. This model provides a prior probability distribution for possible solutions to the inverse problem. The uncertainty in the inverse solution is characterized by a conditional (posterior) probability distribution. A new stochastic simulation method, called conditional coding, approximately samples from this posterior distribution and allows study of solution uncertainty through Monte Carlo techniques. We examine a fracture-dominated flow system, but the method can potentially be applied to any system where formation processes are modeled with a stochastic simulation algorithm.

Utilizing crosswell, single well and pressure transient tests for characterizing fractured gas reservoirs

As part of its Department of Energy (DOE)/Industry cooperative program in oil and gas, Berkeley Lab has an ongoing effort in cooperation with Conoco and Amoco to develop equipment, field techniques, and interpretational methods to further the practice of characterizing naturally fractured, heterogeneous reservoirs. The focus of the project is an interdisciplinary approach, involving geology, rock physics, geophysics, and reservoir engineering. The goal is to combine the various methods into a unified approach for predicting fluid migration.

Integrated reservoir characterization : beyond tomography

In 1992, there was a collaborative effort in reservoir geophysics involving Amoco, Conoco, Schlumberger, and Stanford University in an attempt to delineate variations in reservoir properties of the Grayburg unit in a West Texas CO2 pilot at North Cowden Field. Our objective was to go beyond traveltime tomography in characterizing reservoir heterogeneity and flow anisotropy. This effort involved a comprehensive set of measurements to do traveltime tomography, to image reflectors, to analyze channel waves for reservoir continuity, to study shear‐wave splitting for borehole stress‐pattern estimation, and to do seismic anisotropy analysis. All these studies were combined with 3-D surface seismic data and with sonic log interpretation. The results are to be validated in the future with cores and engineering data by history matching of primary, water, and CO2 injection performance. The implementation of these procedures should provide critical information on reservoir heterogeneities and preferential flow direc...

An integrated approach for characterizing fractured reservoirs

Abstract Experience has shown that fractures and faults within a given array are not all equally conductive or well-connected. To investigate new techniques for locating conductive fracture flow paths, a series of high resolution (1 to 10 kHz) crosswell and single well seismic surveys and interference tests were conducted in a shallow five spot well array penetrating a fractured limestone formation. Two inverse approaches for constructing fracture flow models were applied to the interference test data. Both approaches successfully reproduced the transient pressure behaviour at the pumping and observation wells and indicated a preferential fracture flow path between two wells aligned in an east-northeast direction, the dominant direction of fracturing mapped in the area. Crosswell and single well seismic experiments were performed before and after air injection designed to displace water from the fracture flow path and increase seismic visibility. The crosswell experiments showed that replacement of water with gas produces significant changes in the seismic signal. The single well reflection surveys were able to precisely locate the position of the fracture flow path. This location was confirmed by core from a slant well which intersected a single open fracture at the targeted depth.

Fracture characterization at the Conoco Borehole Test Facility using shear-wave anisotropy

Two multi-component near-offset VSP experiments are used, in conjunction with borehole data, to characterise the subsurface fracture system at the Conoco Borehole Test Facility, Oklahoma. Time delays between the fast and slow split shear-waves are observed to correlate with the heavily fractured sandstone formations. Inversion of the shear-wave splitting estimates is achieved using a Genetic Algorithm which incorporates an anisotropic ray tracing scheme. The inversion results suggest that the fracture orientation is sub-vertical. A method of determining fracture dip using an opposite azimuth VSP method is suggested.

Technical and economic impact of crosswell technology: Progress Report

Conoco`s Crosswell Reservoir Characterization (CRC) team is using Decision and Risk Analysis (D and RA) to evaluate the potential economic impact of the authors` projects. D and RA can be a useful tool for refining the direction of a project and communicating the value of information in economic terms. Initial studies show that CRC has high potential economic value. Three steps were used to analyze the value of the CRC project. A high-level, industry-wide model was developed to look at the technical, application, and commercialization success of the technology. The team considered an infill drilling decision in shallow shelf carbonates as a detailed case. For that case, fully integrating crosswell information with supporting data (well logs, cores, production information, etc.) leads to a three-fold increase in expected value. The third step, a portfolio analysis of all crosswell applications, was initiated, but not completed. Given the potential broad application of CRC and the value obtained by the one detailed case, the technology is expected to have a very high value.

Reservoir Characterization As a Guide to Inversion-a Case Study From a Deep Sandstone Reservoir In Block B, Saudi Arabia

Deeply buried tight sandstones present at least two challenges to the seismic interpreter—p-waves are not sensitive to differences between sand and shale, increasing the non-uniqueness of inversions; and it is difficult to interpret pore fluids in low porosity reservoirs. This study demonstrates how detailed reservoir characterization can be used to determine the most effective method for inversion as well as for calibrating and interpreting the inversion results. This approach was used to map reservoir potential in the Sarah Formation, Block B, Rub al-Khali Basin, Saudi Arabia. Introduction Block ―B‖ is located in the Mukassir uplift of the Rub al-Khali basin. The Sarah reservoir, developed at the top of the Ordovician, is one of the major exploration targets in the area. The Sarah was deposited in a deltaic environment that was subsequently eroded by glacial plucking. The formation comprises very thick units of brown grey, fine–medium, lithic quartz sandstone, quartz sandstone and a few layers of detrital conglomerate.