An Improved Simulation for Interpreting Temperature Logs in Water Injection Wells

An improved simulation for temperature logs (TL's) in water injection wells is described. Improvements based on the reduction of assumptions used by previous investigators are demonstrated by comparison of field data and simulator results showing excellent agreement of TL profiles over the entire well depth. Initial work with the simulator has demonstrated the need for different operational procedures for definite TL surveys in mature wells (those having received significant long-term injection) as compared with young wells. The utility of short-period hot water (SPHW) injection just preceding shut-in as an injection profile amplifying scheme has been investigated in depth through the TL simulator. Finally, sensitivity studies have been run to identify the most important TL parameters and to develop guidelines for improved profiling. Introduction Injection of water into wells is done for three basic reasons: to maintain field pressure, for waterflooding, or to dispose of unwanted brine. For at least two of these it is desirable to know an injection profile. The TL is one way of defining injection profiles and is particularly useful in wells with outside-of-casing vertical flow.As fluid flows down the wellbore, the rock surrounding the wellbore (which is initially at the prevailing geothermal temperature) is heated or cooled by the injection water, depending on its temperature and the rate of heat transfer in the well. This effect is most pronounced in an injection zone where the fluid enters the rock formation, flowing radially outward, and where heat transfer occurs by both convection arid conduction. Except for hot-water and steam injection, the near-wellbore portion of the flooded zone normally will be cooled. Once the well is shut in and fluid flow is halted, the temperature of the well and the surrounding formation starts to return to the original geothermal temperature. The regions above and below the injection zone trend toward the geothermal temperature more rapidly than in the injection zone because of the greater heat transfer in the latter. Thus, by measurement of the wellbore temperature as a function of depth the location of the injection zone can be determined as the region where temperature anomalies occur.The interpretation of TL's to determine injection flow profiles has been attempted previously, both qualitatively and quantitatively. In early studies, quantitative analysis was made by use of Laplace transformations and Bessel function solutions. With the advent of the digital computer, more rigorous analysis can be made with numerical methods to treat heat transfer terms, which had to be removed by simplifying assumptions in the earlier studies.In this paper, we present an improved injection-well temperature simulator of the digital computer variety. This simulator offers an advantage over previous simulators in that wellbore-water heat transfer is modeled both before and after shut-in of the well. This capability allowed us to investigate possible solutions to the problem of lost profile definition in mature injection wells. We have found hot-water injection, for a short period before shut-in, to be a potentially important tool for defining injection fluid profiles in mature wells. SPEJ P. 709^