Infill Drilling in the Rotherwood Field, Harris County, Texas
- William Hurat (consultant)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1991
- Document Type
- Journal Paper
- 502 - 507
- 1991. Society of Petroleum Engineers
- 5.5 Reservoir Simulation, 1.6 Drilling Operations
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- 112 since 2007
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More than 600 billion bbl of oil are estimated to be left behind in abandoned and producing fields. The average field produces only one-third of the oil originally in place. This paper describes a method to determine where this oil exists in odd-shaped fields like the Rotherwood field in Harris County, TX. Conformal mapping is used. The field's configuration is changed to a conformal rectangle where Basset's formula for fluid flow between parallel planes applies. By this means, streamlines are developed showing the directions of oil movement to the producing wells in the reservoir. Where large gaps appear between streamlines, or where any abnormalities are evidenced, an area of unproduced oil exists.
Streamlines of fluid flow from the reservoir to a well are the basis for identifying areas of unproduced oil in a field that are suitable for infill drilling. In the Rotherwood field, Harris County, TX, streamlines were applied to ascertain where these areas exist. The areas of unproduced oil were identified by conformal mapping, a method of determining streamline flow by converting the areal extent of a field to a rectangle and applying Basset's formula, which defines fluid flow between parallel planes. Abnormalities that show up in the mapping, such as discontinuities in the streamline plots or wide gaps, indicate areas of residual oil suitable for drilling infill wells.
Basset's formula1-3 is expressed in x,y coordinates; therefore, a conformal rectangle is necessary. The results obtained on the rectangle can then be converted back to the Rotherwood field proper.
Conformal mapping also has a more general application. It can apply to any fluid flow between wells in a field where faults or shale lines exist or to fluid injections to producing wells in a recovery program.
This paper discusses first the general aspects of the Rotherwood field. This field is in the Cockfield Series A formation at a depth of 5,500 ft subsea. Thirteen wells were drilled, yielding 857,916 bbl of oil. The field, which has an area of 534.2 acres, has ceased to produce and is being considered for secondary recovery.
The purpose of this paper is to determine where the three or four infill wells planned should be drilled.
Fig. 1 presents the Rotherwood field as a whole and includes the original1 wells and their locations. Sixteen bus bars define the limits of the field, with the actual limits represented by broken lines. Fig. 1 also shows the locations of the infill wells.
Fig. 2 is the potentiometric model plot for Rotherwood field. A potential of 1 to 0 is shown plotted for the longest distance in the field, defined by F. The opposite two sides are also plotted from 1 to 0 to define the fluid flux streamline, ?. The probing instrument is connected to a voltmeter that indicates potential changes throughout the model. The model itself has a stainless steel sheeting.
The fluid flux across a fixed potential curve is expressed by
where D=distance along the potential accounting for the curvature of the potential. The slope dF/dD is normal to this potential, determined by the divided difference program developed in the computer. Thus, the product of the ? potential already discussed times Eq. 1 is the flux distribution throughout the model. The proof of this method is that F and ? are normal to one another.
We next turn to Fig. 3, the conformal rectangle, and its mathematical development. The overall potentials and streamline values can determine the resistance, R, of the model expressed by Ohm's law:
R is also defined by
where L=model length and w=model width. When associated with the areal extent of the field, this gives the absolute values of a and b of the conformal rectangle that applies to the Rotherwood field.
Another analytic deduction for transferring a point on the rectangle to the field is given by
Equations 4a and 4b
Details on conformal mapping are reported in Ref. 1, but there it is applied to a circular reservoir.
Table 1 is the cumulative production of the 13 wells that have produced in the field. The productions are expressed relative to a value of unity assigned to the Henry A. Fuchs 2 well, the largest cumulative producing well in the field, to avoid large numbers.
Table 2 refers to the location of the 13 wells on the conformal rectangle, where F and ? refer to their locations in Fig. 2. With Eqs. 4, F and ? are transferred to the conformal rectangle, expressed by ? and ?.
Finally, Table 3 gives the sum total of the 10 wells in Quadrant 3 and shows that this area produced 93 % of the field's total production.
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