Accurate evaluation of remaining oil can be effectively complemented with production logging data in a horizontal well for a successful workover. The pulsed neutron tool has been used for production profile measurements in complex stratified flow regimes in horizontal wells for some years using water flow log and three-phase holdup log measurements. Normally neutron capture sigma data is measured in wells that are shut-in, but this method may be seriously affected by kill fluid invasion. The neutron capture sigma data can also be acquired by a pulsed neutron tool centralized in the borehole while the well is flowing. If the sigma data recorded in a flowing status can be validated for evaluating the remaining oil saturation, then this method can both eliminate the effects of borehole fluid reinvasion and save rig time and operation cost. Furthermore, combined flow profiling and remaining oil evaluation will result in an optimal and effective workover.
This paper gives the details and results of two successful workover cases conducted in oil fields in Tarim basin, China. We used a production logging tool integrated with pulsed neutron tool for velocity and holdup distribution measurements, conveyed with a tractor in slotted liner or a perforated casing completion. It was validated that the neutron capture sigma data acquired in flowing conditions can be used for remaining oil evaluation after the borehole fluid effects were eliminated by data processing. The workover results were unexpectedly good. In one well completed with slotted liner, water-producing intervals were squeezed by a chemical method, resulting in the water cut decreased to 20% from the previous 96% and oil production increased 37 tons per day. In another low-flow-rate well, analysis indicated that shutoff was needed for a water zone in the toe of the horizontal interval. The results confirmed this, with water cut decreased to 86.5% from the former 96% and oil production increased more than 300%. This paper also discusses observations and recommendations.
The Tahe oil field, discovered in 1997, is located in the gobi of the northern margin of the Taklimakan desert in the Tarim basin, northwest China. It consists of 11 independent blocks; Blocks 1, 2, and 9 produce oil from Triassic sandstone reservoirs and others produce from Ordovician carbonate formations. Tahe sandstone reservoirs are found in a low-amplitude anticline with abundant natural energy driven by side and bottom water aquifers. The general reservoir depth varies from 4200 m to 5100 m in different blocks. The formation is highly inhomogeneous, with an average porosity of 19 p.u. to 27 p.u., permeability of 56 mD to 416 mD, and a vertical to horizontal permeability ratio of 1:1.34. The crude oil has a density of 0.91 g/cm3, low viscosity, high wax, and little sulfur content. The formation water is a CaCl2 type with total salinity in the range of 170,000 ppm to 210,000 ppm, water density of 1.1g/cm3, and pH value of 5 (weak acid).