Abstract

The true bed thickness (t) is the actual thickness of a given formation perpendicular to the bedding plane. The value of t depends on the angle and the direction of the dip of the measured formation, as well as the drift angle and azimuth of the borehole. The traditional methods to calculate the parameter t consider only the case of monoclinal beds but not the case of a folded bed, which will cause deviations when the bed dip on the top is different from that on the bottom. To avoid these deviations, this paper shows an approach to calculate the values of t using a folded bed model. The deviations for the monoclinal bed model are positively related to the bed dip, the dip difference and the deviated angle of the wells. A case study from the Cantrell oil field complex in the southern Gulf of Mexico (offshore Campeche) is used to test the folded bed method. The results indicate that this model can yield more uniform spatial change of the values of t, whereas the monoclinal bed model will overestimate the average value of t. Compared to the folded bed model, the maximum relative deviation of t from the monoclinal bed model reaches 22.3% and the maximum absolute deviation of t reaches 34.5 m.

Introduction

In the subsurface of a bed formation, the true bed thickness can be estimated from information gathered from well logs such as the dipmeter and borehole images. The log thickness of a given stratigraphic interval can be thicker, equal to or thinner than that seen in a vertical well drilled through the same stratigraphic section because of well deviation and formation dip. Provided bed dips and well deviations do not exceed a few degrees, the simple vertical-horizontal case is approximated closely enough by the logs. But when deviations and dips do exceed about 10 °, corrections are needed. A previous method of corrections is based on the monoclinal bed model. But this assumption is approximate, because the tectonic events generally cause the beds to be not only tilted, but also folded.

True bed thickness is important in dipping beds and in deviated holes, since reservoir volume and isochore maps depend on these properties and not on the measured thickness. In this paper, a folded bed model is proposed to calculate the true bed thickness in the subsurface utilizing well log data.

Calculation of the Value of t in the Case of Monoclinal Bed

First, the case of a monoclinal bed is reviewed here. Figure 1a shows the case of a vertical well crossing a monoclinal bed. If the well enters the top of the bed at A, and leaves the base of the bed at B, the depth difference between A and B is defined as hd. The true bed thickness of the bed (t) can be expressed as:

Equation (1) (Available In Full Paper)

Figure 1b shows the case of a well deviated at an angle a from the vertical crossing a monoclinal bed.

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