Wellsite Computer Analysis: A Program For Complex Lithologies
Authors: Head Mickey P. 
Published: December 2007

A wellsite computer program is described which solves for porosity and water saturation in formations that have complex porosity and water saturation in formations that have complex lithologies. The reconnaissance portion of the technique combines the responses of a neutron, density, and sonic to identify lithology and formation characteristics. The reconnaissance program can also be utilized as a quick look interpretative technique and as an aid in the selection of parameters for the advanced computation.

The advanced technique employs lithology models determined by the responses of the neutron, density and sonic logs. Total porosity is computed from the neutron-density combination based on porosity is computed from the neutron-density combination based on the correct lithology model, and is checked against the sonic log for the presence of secondary porosity. Water saturation is then computed and the results are printed graphically on a laser recorder.

When gas is encountered, the residual hydrocarbon saturation is computed and is applied to the density log response. By specifying the correct lithology, the density is used to compute porosity which has been corrected for gas effect. porosity which has been corrected for gas effect

Introduction

Direct Digital Logging (DDL), introduced for field use in 1975, has made wellsite computed analysis feasible. Previous Laserlog programs solve known lithology examples that require one porosity device. These models assume that the lithology is known and remains consistent and utilize an induction log and either a density or sonic log to solve for shale corrected porosity and water saturation.

When complex and unknown lithologies are encountered, various combinations of porosity devices are required to solve for both the matrix composition and porosity. The problem that has faced the well operator has been twofold. First, if hand analysis is performed at wellsite, numerous charts and hand calculations are performed at wellsite, numerous charts and hand calculations are required to solve the problem. As additional logs are used, the added data only compounds this already tedious and time consuming problem. problem. Hand held programmable calculators have solved a portion of this problem by aiding the analyst in the tedious calculations. This approach is a vast improvement over hand calculations, but as a practical matter, there is time for only selected intervals to be analyzed. The technique is time consuming because log data must be manually entered for each level of calculation.

Second, if the logs are processed at a remote computer complex, the time required to receive the results can be quite lengthy and can incur additional costs because of the waiting involved.

Previous methods have been used to solve for complex lithology interpretation. These techniques use a neutron-density crossplot approach for lithology and porosity determination with high level logic to handle the effects of shale and residual gas. These are excellent techniques and provide reliable answers, but they are not feasible for wellsite analysis because of their complexity and the need for a highly trained computer log analyst.

The Continuous Crossplot Laserlog, a quick look reconnaissance type program, and the Complex Lithology Laserlog, a program that uses lithology model logic, can perform the tedious calculations in a digital logging truck with both speed and digital accuracy with the results immediately available at wellsite. The programs use neutron and density logs in a crossplot type technique to derive apparent matrix density (Pmap) and crossplot porosity (xp).

In the Continuous Crossplot Laserlog, the addition of a sonic log enables apparent matrix travel time (Tmap) to be computed from a sonic versus neutron crossplot. The values of apparent matrix density (Pmap) Plotted against apparent matrix travel time (Tmap) can be used to identify both the lithology and its characteristics. In the Complex Lithology Laserlog, the sonic is used to check against the neutron-density with logic for lithology model selection.

DESCRIPTION OF THE TECHNIQUE

Figure 1 shows a block diagram of the data from the field tapes to the final results which are displayed graphically on a laser recorder. This discussion follows the sequence shown.