We present a data-analysis approach to determine Archie parameters m and n from standard resistivity measurements on core samples. The analysis method, core Archie-parameter estimation (CAPE), results in computed water saturations that agree well with core-measured saturations. CAPE determines m and n by minimizing the error between computed and measured water saturations. The conventional method minimizes the error in nonphysical quantities. Also, CAPE provides a natural, physically meaningful method of "averaging" Archie parameters, and with an error statistic, aids in zonation of a well or reservoir into different sets of Archie parameters. Finally, we show that the Archie constant a is a weak-fitting parameter, with no physical significance, that can generally be set to unity.

This paper describes a reliability based design of drilling casing and tubing in the load and resistance factor design format. The approach is based on the fundamental principles of limit state design. The paper identifies the limit states of pipe performance in well applications, discusses stochastic modeling of the load and resistance variables, and describes calibration of the design check equations. In the calibration analysis, uncertainties in the various design variables, e.g., kick load intensity and steel mechanical properties, are determined from analysis of the field and laboratory data and represented by appropriate statistical distributions. The reliability based design procedures are complemented by the pipe specifications and quality assurance procedures, also described in the paper. Application of the load and resistance factor design of casing is illustrated by an example problem. BACKGROUND Oil country tubular goods (OCTG) are subjected to a variety of loads during their service lives. These loads originate from various operations, e.g., running, cementing or producing, and accidental conditions such as the kick, or lost returns. Variability of the drilling tubular's strength and loads is well recognized [1, 2]1. The strength uncertainty, for example, arises due to the inherent variability of material properties, workmanship, and handling of tubulars during installation. The load uncertainty is associated with a designer's inability to estimate loads precisely. The objective of design is to estimate the "minimum" strength and the "maximum" load over the life of a tubular and make sure that they are separated by an adequate margin. Traditional design utilizes experience based safety factors, along with a characteristic set of loads and strengths, to assure the safety of tubular design. These procedures work well when a large database of experience supports the design. However, outside of the historical experience, or when new materials and novel applications are considered, for example, deep sour wells, the basis of judgment essential to establish a safety factor is lacking. As a result, the safety of these designs cannot be assured to a known extent. A number of other issues question the validity of the traditional approach to produce optimum casing and tubing designs. Exploration and production well designs consider a distinctly different knowledge base for their load estimation. The consequences of failure of as-intended performance are also different for various applications of a given casing or tubing string. Obviously, the safety margin requirements should also vary. Also, the traditional safety factor approach considers the stress at a selected point to be the appropriate design criterion. However, it can be shown that in well design, as indeed in many other structural applications, capacity of a structural member is better characterized by its gross or total failure behavior, rather than the stress at a selected point. This is due to the fact that load bearing capacity and stress are often not linearly related.

Use of the borehole gravity meter (BHGM) to measure remaining oil saturation is a method new to the industry. The technique is described, and its applicability to Middle East reservoirs is discussed. Results of an extensive error analysis are presented. The method, which we call log-produce-log, consists of running a base BHGM before significant production, and a later BHGM after production. The remaining oil saturation is computed from the difference of the two BHGM measured bulk densities. The Middle East oil fields are ideal for application of this method. Porosities are high, crude is light, and connate water is dense (saline). The method is independent, for all practical purposes, of hole size, rugosity, number of casing strings, shale content, and acidization. The method also has a very large radius of investigation, 50 feet plus, which enables it to sense a far larger volume of the reservoir than any other method. As reservoirs are often heterogeneous in both porosity and fluid saturations, the remaining oil saturation thus determined would be more representative of the reservoir as a whole than any other technique. The disadvantages of the method are the need for a base BHGM log prior to significant production from the zone of interest, and a poor vertical resolution of about 10 feet. Results of a comprehensive and realistic error analysis are presented and show this technique as possibly the most accurate Sor method.