Abstract

The Seria field was discovered in 1929 and the present development concentrates on in fill drainage present development concentrates on in fill drainage and secondary recovery. The field consists of a number of faulted blocks and stacked reservoirs. At the end of 1984, 196 wells out of a total of 750 were closed in for poor production and are candidates for recompletion on poorly drained or undrained reservoirs or for abandonment. Cased hole logs, the Thermal Decay Time (TDT) and the Gamma-ray Spectrometry Tool (GST), have been used to investigate the fluid content behind casing in old closed-in wells prior to abandoning the existing completion. The TDT is used to identify gas bearing intervals, to optimise the GST logging, and the GST is used to identify unproduced oil. Zones identified hydrocarbon bearing from the original open hole electrical logs and confirmed oil bearing with the GST have been successfully completed. New operational and interpretation techniques have been developed and are presented in this paper together with some field results.

Introduction

The Seria field is located in Negara Brunei Darussalam (Figure 1). Since the discovery of the field in 1929, 760 wells have been drilled of which 356 are still producing. The remaining reserves are estimated at about 316 MMbbl: figure 2 shows the production history of the field. production history of the field. The present development activity is aimed at improving the ultimate recovery by infill drainage and secondary recovery. Cased hole logging techniques are used in search of recompletion prospects. prospects.

BACKGROUND

At the end of 1984, 196 - mostly very old - wells in the Seria field were closed-in because of poor production, awaiting recompletion or abandonment. production, awaiting recompletion or abandonment. Most of the closed-in wells have potential recompletion prospects i.e. originally hydrocarbon bearing intervals, which can be identified from the old electrical surveys (ES). However, the hydrocarbon type and saturation are not known because porosity logs are not available; only the ES was logged in the first 550 wells. Various other problems also hamper an evaluation of the remaining recompletion prospects. The Seria field is heavily faulted and the extent of some of the reservoirs, particularly in the central and western part, is poorly defined. The structural geology is under review after an extensive seismic programme carried out in 1984/5. Well performances are often difficult to monitor as some wells are completed over long intervals or produce commingled. A thin gas or water layer can produce commingled. A thin gas or water layer can cause a well to produce with a high gas/oil ratio or a high water cut respectively due to the mobility differences. The extent of the drainage area, the production history and the present fluid content of production history and the present fluid content of many reservoirs is therefore uncertain.

A SOLUTION TO THE PROBLEM

Cased hole logging techniques have to be used to evaluate the reservoirs in these old wells. Two techniques are available, the Thermal Decay Time log (TDT) and the Gamma-ray Spectrometry Tool (GST). The TDT measures the capture cross section (sigma) of the formation (Reference 4). Saturation calculation based on the TDT results rely on the presence of chlorine in the formation water, the presence of chlorine in the formation water, the main "capturer" of thermal neutrons. In Seria, the capture cross section of oil and water are very similar due to low salinity of the formation water (less than 12,000 ppm NaCl). The TDT can therefore not be used to calculate the oil saturation and its use is limited to gas detection. The GST was first introduced in 1982, after field testing of Schlumberger's prototype Inelastic Gamma Tool in 1978. Apart from being used to measure the oil saturation in those wells of the Sari a field where the existing completions had to be abandoned, the GST was also logged to determine residual oil saturation. for a pilot enhanced oil recovery project in the same field. project in the same field. In short the procedure is as follows: Potential hydrocarbon bearing intervals are identified on the old KS logs as being intervals with a low SP (ie. sands) and a high short normal resistivity. Zones indicated to be gasbearing from the TDT are not logged and the oil saturation in the remaining zones is determined fro. the GST. If one or sore sands have substantial amounts of oil left, the original completion interval is abandoned and the well recompleted on shallower intervals, provided the oil production casing is still safe and well cemented. To date, 18 wells have been logged with the GST and all recompleted wells have confirmed the evaluation based on this technique. A more detailed description of the tool, operational procedures and some examples are given below.

PRINCIPLE OF THE GST PRINCIPLE OF THE GST Various publications give a detailed description of the principle of this tool (References 1 and 2) so we will not go into too much detail here. The GST uses a 14 MeV neutron source, which emits neutrons into the formation, inducing gamma-rays through inelastic reactions and thermal capture. The energy spectra of emitted gamma-rays are characteristic for the elements present in the formation, the formation fluid and the borehole. The tool can measure in two modes, inelastic and capture-tau. In the inelastic mode, the energy spectrum of the gamma-rays emitted after inelastic interactions of the high energy and fast neutrons is measured. In the capture-tau mode, the spectrum of the gamma-rays from thermal neutron capture reactions is measured. The gamma-rays are measured and analysed using 256 energy channels from 0 to 8 MeV. The measured spectrum consists of a background and formation spectrum. Each element has a specific gamma-ray energy spectrum, the so-called standard. Figure 3 shows a set of inelastic spectra for various standards and an example of the measured formation spectrum. The relative volumetric presence (or yield) of the various elements is calculated by the surface computer by determining a least squares fit of the standards to the measured spectrum. The GST will measure in the inelastic mode the yields of C, Ca, Fe, O and Si. In the capture-tau mode, the Ca, C1, Fe, H, S and Si yields and the capture cross section are measured. The yields in the capture mode are more accurate than in the inelastic mode but do not include C and O.

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