Cased Hole Formation Resistivity Applications in Alaska

Abstract

This paper describes the theory of cased hole formation resistivity measurements and presents examples demonstrating the diversity of their use. Resistivity behind casing has many applications, from evaluating formations in new wells to monitoring water influx and bypassed hydrocarbons in producing wells. These robust deep-resistivity measurements can be combined with data from traditional cased hole and openhole evaluation tools to provide a comprehensive formation evaluation from behind casing. Example A highlights the use of these measurements in the successful completion of a new Cook Inlet offshore well. Traditional openhole logs were not obtained because of hole conditions and the borehole environment, which also limited traditional cased hole evaluation techniques such as sigma and carbon-oxygen logging. The new resistivity measurements were instrumental to the successful completion of this well as an oil producer with low water cut. Examples B and C, from wells in a mature field, demonstrate the use of the measurements to identify water influx and encroachment, which is vital for reservoir monitoring. Both wells produced water from high-permeability thief zones, which prompted profile modification to limit water production and enhance off-take from zones with lower permeability. The new cased hole resistivity measurements provided valuable data, not available from traditional production logs, to assess sweep efficiency, which will be used to guide recompletion or sidetrack decisions. Introduction Traditional openhole formation evaluation techniques use resistivity or nuclear measurements to identify the location of hydrocarbons within the formations penetrated by the drill bit. In the past, once casing had been set in a well, formation evaluation and monitoring have been restricted to nuclear measurements because it was impossible to measure formation resistivity behind metal casing. Recent advances in electronics and electrode design now allow formation resistivity measurements to be made behind metal casing that can be used for both primary evaluation and reservoir monitoring. The ability to measure resistivity behind casing complements traditional cased hole nuclear evaluation techniques by providing valuable data in low-porosity environments and with greater depth of investigation than previously available. Direct resistivity measurements are also valuable for time-lapse monitoring of reservoir depletion. Three examples of the application of formation resistivity behind casing will be presented from wells located in Alaska. Example A demonstrates the application of measurements from the CHFR* Cased Hole Formation Resistivity tool to new well formation evaluation. In this well, openhole resistivity measurements were not available because hole conditions and formation conditions limited the use of traditional nuclear techniques. Examples B and C demonstrate the application of CHFR measurements to reservoir monitoring in a mature field through comparisons to previous resistivity and water saturation (Sw) results. CHFR Measurement Theory The theoretical basis for resistivity measurements behind casing has been known for a long time, having been patented in 19391. Practically, however, this measurement has not been available until recently because of the large resistivity contrast between metal casing and typical formation materials. The resistivity of metal casing is approximately 2×10-7 ohm-m, while typical formation resistivity ranges from 1 ohm-m to 100 ohm-m2. That contrast is approximately seven orders of magnitude. To measure this large contrast, voltage measurements in the nanovolt range are necessary. This capability has only recently been possible with advances in tool electronics and electrode design.

Mature Field Development - A Review

“…Another cased hole resistivity log measurement was reported by Hupp et al 21 . They proposed the following equation as depletion indicator.…”

Hydrocarbon Potential Monitoring in Gas Sandstone Reservoir Using CHFR and TDT Techniques

“…Because This depletion Indicator (DI) can be plotted versus depth for formation of interest, taking the value 1 as a base line, below which the formation can be considered as it is depleted: RCHFR less than ROH [10][11][12][13][14][15][16][17].…”

Hydrocarbon Potential Monitoring of By-Passed Pay Zones Using TDT and CHFR Logs

“…[1,3] In general case of shaly porous formation containing water and hydrocarbons, water saturation , Sw from TDT is : This depletion Indicator (DI) can be plotted versus depth against formation of interest, taking the value 1 as a base line, below which the formation can be considered as it is depleted: R CHFR less than R OH [10][11][12][13][14][15][16][17]. This technique is based on the fact that capture crosssection of a material is the summation of the crosssection of all its elements multiplied by their respective volume proportions.…”