Borehole Position Uncertainty - Analysis of Measuring Methods and Derivation of Systematic Error Model

Abstract

Summary This article presents a new model for describing well-position uncertainties. An analysis for surveying position uncertainties. An analysis for surveying errors is given that demonstrates that they are mainlysystematic rather than random. The error model, based on systematic errors, compares well withpractical experience. A graph is presented that shows practical experience. A graph is presented that shows typical lateral position uncertainties of deviated wellsfor various kinds of surveys. Introduction During the past 10 years, the uncertainties involvedin determining the true course of a borehole havebecome a cause for concern. The more deviated anddeeper the holes were drilled, the more often were theoperators faced with inexplicable differences betweenvarious surveys made in the same well. As early as 1971, Truex mentioned that possible lateral positionerrors of highly inclined wells could be up to 30 m ata depth of only 2000 m. Two years before that, Walstrom et al. introduced the ellipse-of-uncertainty concept to describe the positionuncertainty, which can be expected with various surveymethods. Experience, however, has shown that theellipse calculated by this random error model isunrealistically small, which is thought to be duemainly to the nature of the statistical error modelused.The essential differences between the existingrandom error model and the model proposed in thisarticle are illustrated by the following simplifiedexample. Consider the straight and inclined part of awell with these directional characteristics: total depthalong hole (AHD or DAH) 2500 m, surveyed at100 stations at 25-m intervals, and all having aninclination of I Delta I = 30 0.5 and an azimuthof A Delta A 90 1.The bottomhole position of this well in north, east, and vertical coordinates easily is found as N = D AH sin I cos A = 0, E = D AH sin I sin A = 1250 m, andV = D AH cos I = 2165 m. The position uncertainty of the bottom of thiswell, according to the error model presented in thisarticle, follows straightforwardly from theassumption that the measuring errors at all 100 stations havethe same magnitude (they are correlated fully).Hence, by simple trigonometry, as sketched in Fig. 1, and In the random error model, however, it is assumedthat the measuring errors vary randomly from onestation to another, which gives them a tendency tocompensate one another. This randomness of themeasuring errors causes the position uncertainty tobe smaller than the former values - in our example, by a factor equal to the square root of the number ofmeasuring stations, which is 100 = 10. JPT P. 2339

Horizontal Well Accuracy and its Effect on Infill Drilling in the Winter Area

“…In summary, analysis of the 1991 to 1999 original horizontal wells and the 2000 infill horizontal wells directional data showed: ( 1) significant azimuth 'offset' disagreement between MWD and gyro surveys in some wells before 7 degrees inclination, (2) systematic azimuth errors between MWD and gyro surveys for both the build sections and the horizontal sections, (3) the systematic error between MWD and gyro surveys in the build sections is similar, but not identical, to the systematic error in the horizontal sections, ( 4) South/North oriented wells did not have less MWD to gyro disagreement than East/West oriented wells. Infill drilling in Section 31 was suspended when the gyro surveys showed some of the original horizontal well pair target locations likely did not have sufficient gap for a short (600 meter horizontal length) infill.…”

All-attitude Gyrocompass Tool Offers Significant Enhancement in Survey Accuracy over the Highest Performance MWD Magnetic Systems

“…As discussed by Ekseth et al 3 , the quality of wellbore position uncertainty has increased significantly over the years owing to major contributions to the subject by Wolff and deWardt's systematic error model concept 4 and John Thorogood's instrument performance models 5 . However, state of the art error modelling is largely based on collaborative work within the SPE Well Positioning Technical Section (WPTS), formerly the ISCWSA, as described in the following of SPE papers; SPE 67616, 90408, 103734 and 105558 [6][7][8][9] .…”

Implementation of a New Risk Based Well Collision Avoidance Method

“…Significant work in this area by Walstrom (1,2,3) , Wolff and deWardt (4,5) , Thorogood (6,7) , Burton (8) , Brooks and Wilson (9) and Williamson (10,11) were used as the basis for this effort. In addition to the literature review we solicited several major operators, service vendors and other ChevronTexaco business units for their existing Collision Avoidance policies.…”