Abstract

A time-lapse (4-D) seismic technique has successfully monitored changes in reflection data due to a pattern steamflood in Duri Field, Central Sumatra. The technique involves acquiring a 3-D survey before steam injection and comparing this baseline dataset to later 3-D surveys recorded during steam injection. The fourth dimension in this technique is calendar time. The Duri baseline dataset was acquired in July 1992, and five additional 3-D surveys have been recorded over the same pattern to monitor steam advances. This experiment represents the first application of this new technology in Indonesia.

Changes in reflection character in the Duri 4-D dataset are significantly larger than those demonstrated in published examples of time-lapse seismic monitoring. These changes are interpreted as a result of combined thermal, presence of steam, and pore pressure effects. After steam injection, a 20 millisecond "sag" develops in the reflection seismic data around the injector well. The seismic reflection "sag" has a radius of 80–110 meters in the most recent monitor survey, recorded 19 months after start of steam injection. The growth of this seismic reflection anomaly in successive monitor surveys shows how the reservoir conditions change with calendar time.

Maps of reflection changes in Duri Field show the progression of pressure, thermal and steam fronts in the pattern. A simple travel time analysis of these changes has yielded insights into the vertical and horizontal heat distribution in the pattern. This analysis shows several "first order" effects of steam injection. An uneven vertical distribution of heat is interpreted from the 4-D data. This is independently confirmed by core permeability and formation damage studies. The 4-D data also show that the dominant thermal effects are moving preferentially to the North and West, which is indicative of directional permeability. Oil saturations for the 4-D pattern were estimated independently with a stream-tube simulation. The simulation results are in general agreement with 4-D difference maps.

Information about the vertical and horizontal distribution of heat in the pattern is needed for steamflood optimization. It allows perforation intervals in injector and producing wells to be judiciously selected or altered. Barriers to steam injection can be identified and infill well locations can be determined.

In addition to mapping thermal effects in the reservoir, the Duri 4-D data have yielded information about the movement of pressure fronts. Before steam injection Duri reservoir rocks have a small (5–10%) percentage of free gas. After steam injection a high pore pressure (above 100 psia) annulus develops outside of the thermal zone. Reservoir pressure is above the bubble point in this annulus and it is identified in the 4-D data by a velocity increase. Pressure effects are transmitted much more quickly than thermal effects in the reservoir. Two months after the start of steam injection the outer radius (100 m) of the high pressure annulus shows the effect of pressure drawdown on producing wells. Later surveys show pressure increases over most of the pattern. Movement of the pressure front may predict thermal and steam front movements at later times into areas of higher permeability.

Analysis of the Duri 4-D data requires integration of engineering, core, and wireline data with a series of 3-D seismic cubes. Recent studies of Duri cores show that significant velocity changes occur as a function of variation in temperature, pore pressure, and fluid phase. Temperature surveys were recorded in observation wells and together with the core measurements allow preliminary calibration of the observed travel time anomalies to temperature isotherms.

Based on the results to date, it appears that 4-D monitoring offers the reservoir engineer an important new tool to optimize the Duri steamflood. This technology may also have application to monitor EOR processes at other CPI fields.