Geophysics For Petroleum Engineers

Geophysical techniques apply the principles of physics for study of physical responses of rocks under passive or active perturbation. Geophysical data acquisition, processing and interpretation are driven by established scientific principles. Data from geophysical tools provide coverage with spatially continuous high density measurements. Well data like cores and well logs provide vertically high resolution measurements at the well location, however, the distribution of wells is sparse and discontinuous. The detailed spatial coverage from geophysical data are calibrated with analysis of well logs, pressure tests, cores, geologic depositional knowledge and other information from appraisal wells. The methods use high precision sensors that measure the properties on the earth's surface, in oceans, in wells and from the air, also from satellites. They measure changes of physical properties and calibrate the measured geophysical attributes with rock properties. The data play important role in developing a gross reservoir model. The reservoir architecture or structure and the reservoir rock and fluid properties are derived from the analysis and data integration. Other reservoir properties that can affect geophysical measurements are density, oil viscosity, stresses, and fractures. The interpretation has inherent ambiguity or multiple interpretations. Geophysics contributes to reservoir characterization, reservoir monitoring and its management by adding maximum value in improving production plan and by minimizing risk e.g., risk of dry hole, risk of blow out, risk of in-efficient recovery process, among others. Multiple geologic parameters are assessed with the same geophysical data.

Geophysics for Petroleum Engineers focuses on the applications of geophysics in addressing petroleum engineering problems. It explores the complementary features of geophysical techniques in better understanding, characterizing, producing and monitoring reservoirs. This book introduces engineers to geophysical methods so that they can communicate with geophysicist colleagues and appreciate the benefits of their work. These chapters describe fundamentals of geophysical techniques, their physical bases, their applications and limitations, as well as possible pitfalls in their misuse. Case study examples illustrate the integration of geophysical data with various other data types for predicting and describing reservoir rocks and fluid properties. The examples come from all over the world, with several case histories from the fields in the Middle East. Introduces geophysical methods to engineers Helps understanding, characterizing, producing and monitoring of geophysical techniques Updates the changing needs of reservoir engineering

Geophysical methods are being applied that are specifically relevant to the exploration of and production from unconventional reservoirs. While many of the techniques have common applications for both conventional and unconventional reservoirs, there are also some significant differences in focus. Much of the unconventional reservoirs are from shale formations. Characterizing of fracture system in such reservoirs is of prime importance not only to identify the “sweet spots” for well placement but also for optimum drilling and production from such fractured reservoirs. Combining conventional and microearthquake seismic data has proven to improve the characterization process. Another factor for drilling through shale reservoirs is the need for stimulation through hydraulic fracturing. Use of microseismic data for monitoring the frac process has gained prominence in recent years. Different types of design to acquire such geophysical data and to utilize them for multistage fracking process as well as their integration with the conventional seismic data have been developed.

The process of drilling an oil or gas well requires knowledge of all geologic features expected to be encountered along the way—from the surface of the ground to the target reservoir. Thus, in addition to steering the well so as to intersect hydrocarbon-bearing reservoirs, the reservoir engineer must assure to a reasonable degree of confidence that the well drills successfully and safely to the target. Geophysical measurements help ensure a successful drilling program. 3D seismic provides a picture of the subsurface from the surface to the target.

Origin of petroleum begins with the formation of organic matter, burial of organic matter in a basin maturation of the organic content with pressure temperature at burial depths. Petroleum system includes source rocks, reservoir rocks, reservoir traps, migration paths, seals etc. Hydrocarbons mature in source rocks migrate into reservoirs. Reservoir rocks are containers of hydrocarbons with sufficient interconnected pore spaces, these are sedimentary rocks clastic (sandstone shale), carbonate rocks. Migration pathways for matured hydrocarbons- migration is in separate phases from higher potential to a lower potential, from deepest to the shallowest. The hydrocarbons migrate into different types of petroleum traps such as anticlinal, fault, salt related stratigraphic traps. Reservoir seals are rocks with low permeability drapes hydrocarbons traps to ensure that entrapped fluids do not escape. Integration of disciplines geology, geophysics, petrophysics, is the key to predicting reservoir geometry, volume, in assessment of reserves.

In most oil exploration and production problems, we deal with limited and incomplete data. We are constantly trying to extrapolate information from sparse measurements, for example, sparse well data and limited core measurements on the one hand and large volumes of seismic data with limited spatial resolution on the other hand. We resort to statistical methods to accomplish the data extrapolation and the integration of diverse data sets in constructing a coherent and meaningful model of the subsurface. Traditional statistical methods both for spatial and temporal extrapolation have been used in E&P for several decades. One of the main uses of statistics has been for reservoir characterization through integrating information and data from various sources with varying degrees of uncertainty such as log, well tests, and seismic data. Other applications include establishing relationships between measurements and reservoir properties, and reserve estimation and oil field economics along with the associated risk factors.

Petrophysical analysis of well logs and cores provide information about formation rocks and fluids in the borehole. Various types of well logs measure different properties in the well. Analysis of the data determines the volume of hydrocarbons present in a reservoir, and its potential to flow through the reservoir rock into the wellbore. This helps us to understand and optimize the producibility of a reservoir. When oil and gas wells are drilled, physical property measurements are taken using specialized geophysical instrument packages: either on wireline cables after the drill pipe has been removed (wireline logs), or from the borehole while drilling with instruments attached to drill collars (LWD).