Wednesday, April 4, 2018

Saturation Monitoring With the RST Reservoir Saturation Tool

The RST Reservoir Saturation Tool combines the logging capabilities of traditional methods for evaluating saturation in a tool slim enough to pass through tubing. Now saturation measurements can be made without killing the well to pull tubing and regardless of the well's salinity.

Determining hydrocarbon and water saturations behind casing plays a major role in reservoir management. Saturation measurements over time are useful for tracking reservoir depletion, planning workover and enhanced recovery strategies , and diagnosing production problems such as water influx and injection water breakthrough.

Traditional methods of evaluating saturation - thermal decay time logging and carbon/oxygen (C/O) logging - are limited to high-salinity and nontubing wells, respectively. The RST Reservoir Saturation Tool overcomes these limitations by combining both methods in a tool slim enough to fit through tubing. The RST tool eliminates the need for killing the well and pulling tubing. This saves money, avoid reinvasion of perforated intervals, and allows the well to be observed under operating conditions. Moreover, it provides a log of the borehole oil fraction, or oil holdup, even in horizontal wells. 

To understand the operation and versatility of the RST tool requires an overview of existing saturation measurements and their physics.

The Saturation Blues

In a newly drilled well, openhole resistivity logs are used to determine water and hydrocarbon saturations. But once the hole is cased, saturation monitoring has to rely on tools such as the TDT Dual-Burst Thermal Decay Time tool or, for C/O logging, the GST Induced Gamma Ray Spectrometry Tool, which can "see" through casing.

The Dual-Burst TDT tool looks at the rate of thermal neutron absroption, described by the capture cross section gamma of the formation, to infer water saturation. A high absorption rate indicates saline water, which contains chlorine, a very efficient, abundant thermal-neutron absorber. A low absorption rate indicates fresh water or hydrocarbon.

The TDT technique provides good saturation measurements when formation water salinity is high, constant and known. But oil production from an increasing number of reservoirs is now maintained by water injection. This reduces or alters formation water salinity, posing a problem for the TDT tool. 

In low-salinity water (less than 35,000 parts per million), the tool cannot accurately differentiate between oil and water, which have similiar neutron capture cross sections.

When the salinity of the formation water is too low or unknown, C/O logging can be used. 
C/O logging measures gamma rays emitted from inelastic neutron scattering to determine relative concentrations of carbon and oxygen in the formation. A high C/O ratio indicates water or gas-bearing formations. 

The major drawback to C/O logging tools has been their large diameters. Producing wells must be killed and production tubing removed to accomodate tools with diameters of nearly 4 in. [10 cm] . In addition, the tools have slow logging speeds and are more sensitive to borehole fluid than formation fluid, which affects the precision of the saturation measurement.

As Easy as RST

The RST tool directly addresses these shortcomings and can perform either C/O or TDT logging. It comes in two diameters - 1 11/16 in. (RST-A) and 2 1/2 in. (RST-B) - and can be combined with other production logging tools. 

Both versions have two gamma ray detectors. In the RST-A tool, both detectors are on the tool axis, separated by neutron and gamma ray shielding. In the RST-B tool, the detectors are offset from the tool axis and shielded to enhance the near detector's borehole sensitivity and the far detector's formation sensitivity. This allows the formation oil saturation and borehole oil holdup to be derived from the same RST-B C/O measurement. 

Locating Bypassed Oil

In early 1992, ARCO drilled and perforated a sidetrack well in area of Prudhoe Bay undergoing waterflooding. Less than six months later, production was 90% water with less than 200 BOPD, as expected. The original perforations extended from X415 to X440 ft. C/O logging measurements were made in the shut-in well with three different tools - the RST tool and two sondes from other service companies.

The RST results confirmed depletion over the perforated interval (Tracks 2 and 3). Effects of the miscible gas flood sweep are apparent throughout the reservoir. The total inelastic count rate ratio of the near and far detectors indicates qualitatively the presenc of gas in the reservoir. In addition, differences between the openhole fluid analysis and the RST fluid analysis were assumed to be gas.

One potential bypassed zone, A, was identified from X280 to X290 ft. A second zone, B, based on the openhole logs and a C/O log from another service company, was proposed from X220 to X230 ft. The RST log shows zone B to contain more gas and water than zone A.

After assessing the openhole logs and the three C/O logs, ARCo decided to perforate zone B. The initial production was 1000 BOPD with a 75% water cut. Production declined to 200 BOPD with more than 95% water cut in a matter of weeks. The decline prompted ARCO to perforate zone A, commingling production from earlier perforations. Production increased to an average of 600 BOPD and the water cut decreased to 90%. Subsequent production logs confirm that zone A is producing oil and gas and zone B is producing all of the water with some oil.

Modes of Operation

Flexibility is a key advantage of the RST tool. It operates in three modes that can be changed in real time while logging:

  • inelasitc-capture mode
  • capture-sigma mode
  • sigma mode

Inelastic-capture mode: The inelastic-capture mode offers C/O measurements for determining saturations when the formation water salinity is unknown, varying or too low for TDT logging. In addition to C/O logging , thermal neutron capture gamma-ray spectra are recorded after the neutron burst. Elemental yields from these spectra provide lithology, porosity and apparent water salinity information.

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