Increasing Oil Production For An EOR Project

Cased-hole environments limit the technical options for a viable logging program to support reservoir management decisions. For decades cased-hole reservoir monitoring has been commonly used to provide the necessary information for decision-making on EOR projects. This information includes formation porosity and hydrocarbon saturations and how they vary both vertically within the reservoir and laterally from injection to production wells.

Pulsed-neutron tools are an ideal solution for measuring porosity and fluid saturations in a cased-hole environment. Multidetector pulsed-neutron tools offer increased accuracy with the ability to solve for threephase saturations in challenging reservoirs. Pulsed-neutron logging tools can be run in injection wells, production wells and unperforated monitoring wells, creating a picture of the formation and fluids as they travel through the reservoir. In addition, monitoring or logging a well at scheduled intervals can depict how the fluids are changing with respect to time.

Pulsed-neutron technology

The Kinder Morgan Scurry Area Canyon Reef Operators Committee (SACROC) Field is a mature Permian- age carbonate reservoir in West Texas with a complex fracture network of limestone and dolomite vugs. SACROC was under waterflooding for many years before being switched to a super-critical CO₂-type EOR project. This also is described as a miscible flood, which mixes with the residual oil, allowing increased oil production and recovery. The original oil in place was estimated at about 2.8 Bbbl of oil, with a cumulative production of 1.2 Bbbl. Production from CO₂ flooding is about 30,000 bbl/d.

Originally, the SACROC Field was estimated to have a 90% oil saturation. Conventional infill drilling and production brought the oil saturation down to roughly 60%. Maximum waterflooding recovery methods can bring this down to an estimated 30% oil saturation under ideal conditions. EOR methods such as CO₂ can ultimately reduce the oil saturation to less than 20%.

The field consists of three types of wells—CO₂ injection wells, dedicated monitor wells and production wells. Logging injection wells over time can cast light on which zones are taking the most super-critical CO₂ and can give an indication of the sweep efficiency of the CO₂ flood and how it is progressing through the reservoir. The CO₂ flood sweep efficiency is affected by changes in porosity, horizontal and vertical permeability changes, and the fracture network as well as other heterogeneity factors. Logging the production wells over time shows the changes in the reservoir, production efficiency and rates at which the zones are changing relative to water, oil and CO₂. This information also will clarify overall sweep efficiencies and inefficiencies that may be addressed.

Maximizing recovery

Knowledge of which zones the injected CO₂ is flowing through and the residual oil saturations allows the operator to tailor the CO₂ flood. Maximizing recovery, even by a few percentage points, of the remaining 1.6 Bbbl can greatly improve the return on investment.

CO₂ saturation would historically be calculated using pulsed-neutron sigma-decay methods, but with mixed and unknown salinity due to a history of flooding operations, sigma-based saturations have high uncertainties, thereby requiring a different measurement method. The oil saturation from a traditional carbon-oxygen log also would be affected by the miscible CO₂ in the oil phase. In addition, porosity is a key component to all saturation and volume calculations. Measuring the increased porosity from the recent acid treatments must be done to calculate accurate saturations.

Accurate saturation, porosity calculations

The Reservoir Monitor Tool 3-Detector (RMT-3D) pulsed-neutron device solves for water, oil and CO₂ saturations in the reservoir using three independent measurements: sigma; carbon-oxygen; and saturation gate (SATG), a proprietary gas detection measurement. Carbon-oxygen elemental yields-based saturation calculations are used to determine the saturation and volumes of water and oil in the formation. Carbon- oxygen is independent of formation water salinity but must be corrected for the presence of gas or CO₂. Sigma-based measurements can be used to determine gas or CO₂ volumes and saturations but rely on accurate knowledge of formation water salinity. In reservoirs where the formation water salinity is low, mixed or unknown, sigma-based measurements have high uncertainty.

The RMT-3D SATG ratio gas-saturation measurement has reduced salinity as well as lithology dependency, which allows accurate CO₂ saturation calculations and provides the CO₂ volume needed to correct carbon-oxygen results. In addition, porosities can be calculated using pulsed-neutron-based inelastic and capture ratios, which are similar to openhole neutron and density porosity measurements.

Providing necessary insight

This logging program provided the necessary insight into what was occurring downhole to make decisions for the project moving forward. Kinder Morgan has been able to reduce cost and risk while also improving production and recovery by switching to the cased-hole RMT-3D pulsed-neutron logs for both new and existing wells in some areas. For new wells traditional openhole wireline logs were used, which increased drilling rig time and costs. The RMT-3D tool was deployed after the casing had been set and the rig moved off location, thus saving rig time. On existing cased-hole wells the RMT-3D tool was able to solve for porosity, water, oil and CO₂ saturations. Monitoring injection wells, dedicated monitor wells and production wells over time has enabled Kinder Morgan to maximize the sweep effi - ciency of the CO₂ EOR project by increasing oil production and recovery.

Accurate formation evaluation of complex reservoir fl uids would have been diffi cult if not impossible in years past. Acidization treatments to increase porosity, mixed and unknown formation water salinity, and super-critical CO₂ injection that is miscible with the residual oil would have led to high uncertainties when using traditional sigma and carbon-oxygen pulsed-neutron technology. The multidetector pulsed-neutron RMT-3D tool was able to solve for three-phase saturation and monitor this complex EOR project.

In addition to porosity and fluid saturations, pulsed-neutron tools also have been used for many other purposes, including openhole log replacement, basic lithology using both the inelastic and capture elemental yields, gravel-pack evaluation for voids in the pack using silicon activation, hydraulic fracture placement using inert tracer proppants and detecting water flow channeling behind casing using oxygen activation.

For further information, a similar case study is investigated in SPE paper 165230.

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