Coring and Core Analysis (Introduction)

Introduction

Reservoir rocks saturated with hydrocarbons are complex on both a macroscopic and microscopic scale. The complexity of both rock and fluid properties controls the initial quantity and distribution of hydrocarbons and the rate of flow of these fluids within the reservoir, as well as the volume of hydrocarbons recovered. A sample (core) can be taken to recover a portion of the reservoir rock, so that it may be examined firsthand and tested in the laboratory. Such direct physical measurements furnish both geological and engineering information and guide the decisions affecting both the cored well and subsequent wells in that area.

Reservoir rock characteristics vary both vertically and areally. Under the best of circumstances, the volume of rock recovered through coring, even when all wells at a location are cored, is small when compared to the rock volume of the reservoir. Although wells should be cored and data gathered to define reservoir rock properties in all dimensions, in many cases insufficient core data are taken to do a complete evaluation. Well logs then become helpful. A synergism exists between direct physical measurements on core samples and what can be learned from downhole well logs and other well-test data. Good engineering recognizes this fact and utilizes the strengths of all evaluation tools to make the best possible development decisions.

Core analysis is the name given to the test procedures and data collected on core samples. A variety of information and data may be obtained via measurements of physical and chemical properties, visual observations, and photo-graphs. The two major categories of core analysis are conventional core analysis, with associated complementary data, and special core analysis. Other specialized studies are often made that do not fall neatly within either of these categories but are important to both engineers and geologists.

Conventional core analysis yields the most basic data about a reservoir, such as:

the presence or absence of hydrocarbons;

the storage capacity (porosity), the flow capacity and its distribution (magnitude and profile of permeability);

the lithology and texture of the formation.

These data, and complementary measurements made on request, can be available for use within hours or days after a core is recovered, since laboratories are normally close to the area where the cores are cut.

Special core analysis tests are more complex and the data furnished are of wider diversity. Typically, they will require longer core preparation and testing times and more specialized and expensive equipment. Large quantities of data are captured on the more sophisticated tests, and computer assistance is routinely used to calculate results. The increased time factor should be recognized and accounted for when planning a project. Special core analysis tests may be divided into static and dynamic measurements.

When cores are removed from the reservoir environment they are subjected to alterations of pressure and temperature. These alterations cause changes in bulk and pore volume, reservoir fluid saturations, and, in some cases, reservoir wettability (preference of the rock for water or oil). The effect of these changes may be negligible or substantial, depending upon the rock and reservoir fluid characteristics, as well as the rock property being investigated. In conventional core analysis (with the exception of unconsolidated rock where overburden pressure effects are included) these effects are normally ignored. In many of the special core analysis test sequences, both pressure and temperature are important, and laboratory equipment and techniques are designed to simulate reservoir conditions.

Objectives of a Coring Program

Coring has both engineering and geologic objectives, and these should be carefully defined before coring commences. In some cases the objectives conflict, and it is impossible to satisfy all requirements on a given well. The objectives that are established will affect the selection of both the coring fluid and the coring device to be used. The decision will also affect the choice of a suitable core handling and preservation technique and will define most measurements required.

Engineering Objectives

The engineering objectives of a coring program include:

defining areal changes in porosity, permeability, and lithology-the data needed for estimates of reserves and mathematical models;

defining reservoir water saturation (this requires the use of coring fluids that are oil base or that do not invade the rock,* and that the core be taken from above the water transition zone);

assisting in the definition of reservoir net pay;

providing information for calibrating downhole logs as well as the measured values of electrical properties that will be used to improve log-calculated water saturations;

acquiring data on the magnitude and distribution of reservoir residual oil saturation (this is normally needed in enhanced oil recovery studies and utilizes either a pressure or sponge core barrel);

providing core material from which petrographic studies can be made to define clay type and distribution; these yield subsequent guidance in selection of nondamaging drilling, coring, and completion fluids;

acquiring rock samples for special core analysis studies, including relative permeability, capillary pressure, and formation wettability tests;

providing data on porosity, as well as on horizontal and vertical permeability distributions, for use in the design of well-completion programs to ensure that oil is not isolated and left behind the pipe.

(One company suggests that an oil-base mud, with physical properties close to those of the reservoir oil and with no surfactants and low API filter loss, be used to determine residual oil saturation and to preserve wettability.)

Geologic Objectives

The geologic objectives include:

defining gas-oil and oil-water contacts, formation limits, and type of production expected;

providing core data from which the depositional environment can be deduced, including grain size and grain size sequences; vertical sequence of facies; sedimentary structures (ripples, cross bedding); biogenic structures (root zones, burrows); diagenetic alterations (cementing, secondary porosity, secondary mineralization);

permitting a visual study of the frequency, size, strike, and dip of fractures. This requires that fracture studies be undertaken and may require the availability of an oriented core;

retrieval of oriented cores so that directional permeability trends can be ascertained; this applies to both fractured and to nonfractured samples; acquisition of samples of nonreservoir rock in exploration areas so that source bed studies can be made.