Resistivity Analysis to Determine Productive

¶ … resistivity analysis to determine productive zones of hydrocarbon. The summary discusses how the presentation details Archie's equation to determine resistivities and how to read logs to find productive zones. It discusses how to use water saturation to differentiate between hydrocarbon zones and low porosity zones. Finally, this paper summarizes how the presentation combined the steps and processes of resistivity analysis by applying this method to real world scenarios.

This presentation is a guide to performing resistivity analysis to evaluate the presence of hydrocarbons which are the main components of oil and natural gas. The method used to detect hydrocarbon is electrical resistance of formation and this is the basis for the resistivity analysis. The presentation begins by discussing Archie's equation as it pertains to solving the resistivity of water, rock with water, and rock with water and oil. The presentation moves on to discuss log interpretation to find areas that contain hydrocarbons. It describes how to use the graphs and logs to determine permeable zones and productive zones. The presentation concludes by discussing how to calculate water saturation to differentiate between hydrocarbon zones and low porosity and includes sample data to practice this.

The presentation introduces the need to find the resistivity of water, rock with water, and rock with water and oil; it also explains how this is resistivity analysis will help determine areas where hydrocarbons are present. It begins by reintroducing Archie's equation, which relates the electrical conductivity of rock that is in situ to its water saturation and porosity. The variables in Archie's equation are identified and defined. The water resistivity (Rw), rock resistivity (R0), formation factor (F), cementation exponent (m), cementation factor (c), true formation resistivity (Rt), resistivity index (I), saturation exponent (n), water saturation (Sw) and rock saturation (Cw) are all discussed and the presentation explains where this data can be retrieved, such as from deep resistivity measurements for Rt or nearby wet zones for Rw. This portion of the presentation walks through each of the equations necessary to solve these variables and describes what each variable is used for in the resistivity analysis. Charts are also included to serve as a visual representation of the definitions of the resistivities. This section of the presentation details how to solve Archie's Equation as it is the key to the formation evaluation.

The next section of the presentation focuses on a sample problem where specific sample numbers are provided to find the number of barrels of oil in situ and the number of cubic feet of gas in situ. It goes through the steps of how to solve the sample problem using real variables to test the equations. It specifies the reservoir pressure (Pr), reservoir temperature (Tr) and gas deviation factor (Z). The numbers are substituted into the equation to determine barrels of oil in situ (N) and cubic feet of gas in situ (G).

The presentation moves on to discuss basic log interpretation procedures. A background is given for the three basic logs that are required for interpretation. The first log indicates permeable zones and includes data for SP and GR, the second is the resistivity log that includes data for induction and laterolog, and the third is the porosity log that includes data for density, neutron, acoustic and other elements. The purpose of analyzing these logs is to determine the location of potential hydrocarbon producing zones and how much hydrocarbon, as either oil or natural gas, they contain. This section of the presentation also discusses the methods that can be used to locate these potential hydrocarbon producing zones, the methods used to obtain saturations and the methods used to obtain porosities. An idealized log set and instructions are provided to show how to read the three logs for permeability, resistivity and porosity.

This presentation transitions into a more detailed explanation of how to select productive zones. Using the idealized log set containing information on permeable zones, resistivity and porosity, the presentation describes how to identify the shale base line and then look at deviations from it on the spontaneous potential curve to locate permeable zones. It also describes how to search the resistivity curve for high resistivities which can indicate either hydrocarbons or low porosity. The presentation further describes how to differentiate between hydrocarbons and low porosity by determining the water saturation.