Wireline (Pty) Limited is based at Mooinooi near Rustenburg from where the company offers contract wireline logging services to the region. It can trace its heritage back to BPB Instruments, who first brought logging services to South African explorers in 1976, then Reeves Wireline Services who were in turn acquired by oilfield service giant Weatherford. That company continued to upgrade the Reeves tooling which is regarded as the best mineral logging equipment in the world.
Weatherford pulled out in 2014 leaving local management to purchase the company. Wireline’s tooling and pedigree set them apart from the other service providers. It does not always matter but in oil & gas projects, borehole compensation for well diameter, mud weight and formation chemistry is critical. Wireline is honoured to work for Booi Brothers Petroleum, of course, but also delighted to be part of such an ambitious team in a new and exciting market.
Wireline is owned by Allan Bennett who is represented on the Limpopo project by Marcus Chatfield. Marcus logged his first borehole in 1982 and has travelled the world in his search for new applications for mineral logging technology. He became manager of the company’s South African branch in 1986, then manager of Reeves worldwide mineral logging services in 2001 then manager of Weatherford’s operations in 2004 until 2007 when he left to start his own business. The new company, called Wireline Workshop, grew rapidly and, keeping the link with Reeves tooling, was purchased by its friends at Wireline in 2022.
Marcus has produced a book on borehole logging as well as a 60 episode internet blog called Wireline Workshop which enjoyed a readership of over 6,000 loggers and geoscientists throughout the world. It is offered free of charge from Wireline Chronicle’s web
site. It will be converted into a book in 2025. The project in Limpopo is regarded as an oil & gas play. It will involve some extra risks and the need for greater accuracy than most mineral logging projects. Boreholes tend to be of a larger diameter since they are not just conduits of knowledge but conduits of oil and gas production. The borehole is referred to as a well and there is a fairly standard procedure involved in the wireline logging operation.
Firstly, the top section of the well must be cased and grouted in order to isolate the gas and oil within the pay zone and well below potable water aquifers. The grouting job is checked by Wireline using a Cement Bond Log or CBL. The casing rings when struck by a sonic wave unless it is deadened by the presence of cement filling the annulus.
When the driller reaches target depth or, sometimes, an intermediate depth, a suite of tools is run. Firstly a Three-Arm Caliper/ Gamma/Temperature sonde is run. This is referred to as a first-run tool. It checks wellbore conditions before expensive or radioactive sondes are deployed.
The next step is to run a combination of porosity log. We convert sonic, density and neutron logs to estimates of porosity based on a sandstone matrix which comprises just quartz and water. Using density, for instance, we can say that 100% quartz or zero porosity will have a density of about 2.65gm/cc. A porosity of 100% will have an electron density of 1.11gm/cc. So we apply a sliding scale. The three logs will only agree in clean sandstone. In the Limpopo well, quartzite dominates and that gives us a problem because its matrix is denser than sandstone’s at about 2.70gm/ cc, and this is the average of a range. Some considerable analysis is required to solve that problem. If the porosity is full of gas, we get a gas crossover. Density porosity exhibits a
high value because the density falls. Neutron porosity becomes low because there are fewer hydrogen atoms in the pore space.
A resistivity sonde is run to supplement porosity. It also says something about the contents of the pores. Water is conductive, oil is resistive. So we can determine the presence of oil. The dip and direction of rock mass bedding, layering and fractures intersected by a borehole are measured using acoustic and optical televiewers. In mud-filled wellbores, where the centralised detector’s view is obscured, we deploy a resistivity-based formation dipmeter sonde. Its detectors are pressed against the borehole wall and so it avoids the mud problem.
Wireline logs that describe the wellbore column or detect groundwater ingress points such as the spinner flowmeter, fluid sampler and fluid conductivity tools are also available. Then there are the exotic measurements offered by NMR (nuclear magnetic resonance), BHR (borehole radar and SGR (spectral gamma ray). Most logs will be attempted in the study well, currently being drilled.
In the Limpopo project, the driller does not take core samples. The geologist is dependent on the wireline logs to describe rock formations intersected by the bore and, in particular, to recognise hydrocarbons in pores and fractures. Although not a standard oilfield tool, due to the practice of introducing mud to overbalance the well and improve cutting returns, the optical televiewer may be run in the well if the fluid is clean. It has already identified gas ingress visible as black clouds on the borehole wall. Wireline’s goal is to provide a locally-based slimline mineral logging service of such quality that its client is able to avoid importing high cost conventional oilfield technology.
