Reservoir Characterization

Using Production Performance

Case History: TXL Unit (West Texas)

Tom Blasingame

Department of Petroleum Engineering

Texas A&M University

College Station, TX 77843-3116

(979) 845-2292 —

TXL Unit Example: Introduction

The TXL (Clear Fork) Unit is a typical low per-meability, shallow-shelf-carbonate reservoir lo-cated in Ector County, TX. This reservoir has been under primary production at 40-acre spac-ing and was reviewed for infill/waterflooding.

Work to date—production data analysis:

Reserves were estimated using well logs

EUR Analysis: (q/p) versus Np

Decline Type Curve—Closed Reservoir

Mapping of reserve estimates and reservoir properties.

Comparison Plot: OOIPlog versus OOIPWPA

TXL Unit Example: Well TXL-S 5601—History

Oil flowrate is allocated based on periodic tests. The rate "stabilization" at approximately 1 STB/D is reasonable.

TXL Unit Example: Well TXL-S 5601—EURPI Plot

(q/p) trend decays rapidly (reflecting the rate-time de-crease)—this is probably a remediation candidate.

TXL Unit Example: TXL-S 5601—Data Edit Plot

An excellent trend of (q/p) vs. —virtually all of the data adhere to the dominant trend.

TXL Unit Example: TXL-S 5601—WPA Plot

Excellent match—skin factor indicates significant stimula-tion. From EUR analysis, late-time production decay sug-gests remediation, WPA indicates minor deviation from the material balance trend—well interference? (probably not).

TXL Unit Example: EURPI Map

EUR-Map — Contour Interval 25,000 STB

EURPI mimics OGIP—EURPI shows a "ridge" along the right-hand-side of the map. This is trend is consistent with the geologic description.

TXL Unit Example: OOIP Map

OOIP-Map — Contour Interval 400,000 STB

OOIP also shows the same trend along the right-hand-side of the map. The northeast corner has the very highest re-serves, west-southwest region has the lowest reserves.

TXL Unit Example: kh Map

kh-Map — Contour Interval 10 md-ft

kh mimics OOIP/EURPI—major issue is that reserves and kh are lowest in the west-southwest region—this discouraged the operator from infill drilling in this region.

TXL Unit Example: OOIP Comparison

This plot provides a comparison of the OOIP computed using well log analysis versus the OOIP computed from decline type curve analysis. Note that OOIPlog is always greater than OOIPWPA.

Summary:

The TXL (Clear Fork) Unit is a very large dol-stone reservoir of moderate to low reservoir flow capacity. The production analysesconfirm a "ridge" of higher flow capacity, with lower, more dispersed trends on the flanks of this ridge.

This study was conducted because the operator wantedto characterize the reservoir without bias of reservoir simulation. The production-based analyses and the analyses performed using well logs compare reasonably well, al-though significant differences do exist.

In particular, we note thatOOIPlog is always greater than OOIPWPA—though this could (rea-sonably) be accounted for by considering the basis of the well log-derived reserve estimates.

Reservoir Characterization — TXL Unit(West Texas) — Slide 1