Q. In your previous interview, you explained how Schlumberger wireline tools were just as capable of being run in heavy oils as light (conventional) oil. Have you had to go back to basics when interpreting results obtained in a heavy oil environment?

A. We always use basic physics to examine what level of response we get from reservoir fluids. There are certain tools which perform better in conventional oils than heavy oils. Indeed, we would be remiss in our responsibilities to our clients if we did not acknowledge and mitigate for this.

To measure a heavy oil reservoir using light oil techniques will not produce accurate results. We need to take into account the characteristics of heavy oil molecules. We do this by adjusting the methodology employed, as well as subtly and yet significantly adjusting the measurements obtained from the different tools.

Q. So I assume that certain tools are more suited for heavy oil measuring than others?

A. I have previously described the application of some particular wireline tools that have been useful in heavy oil reservoirs. They are the reservoir saturation tool (RSTPro*) where we use carbon/oxygen techniques; a dielectric measurement with the electromagnetic propagation tool; the magnetic resonance (either the CMR-Plus* or MR Scanner*) tool; the sidewall core tool; and the pressure and fluid sampler tool. All these technologies have applications in traditional light oils, as well as capabilities in quantifying heavy oil and the reservoirs in which they occur. However, as you might expect, their suitability in heavy oil as opposed to light oil environments, varies considerably.

Q. What’s the best method to understand total pore space?

A. Magnetic resonance tools measure total pore space when the pore space is filled with water or light oils, but when measuring heavy oil we combine it with another pore space device, the bulk density tool. The tool combination, considering the special nature of heavy oil, provides the most accurate measurement method.

Q. Is it acceptable to take the results of one of these surveys in isolation, or do they needed to be combined to the results of other surveys to be useful?

A. With any sort of evaluation of a reservoir (heavy oil or conventional) it is always better to take a spread of measurements and look at them synergistically in order to obtain a full picture of what is happening in the formation. We call this the “cross comparison of results”.

Q. Are you able to advise customers on the types of tools and production methods they could use to maximize their resource.

A. Yes. Not only are we able to evaluate the reservoir parameters but we also provide a proposal of the best means of completing and producing their reservoir.

Q. How accurate are the measurements that these tools give you in heavy oil reservoirs?

A. Our tools are cutting edge and the measurements they produce are widely considered the best that the industry has at its disposal. However, we never cease to examine how to improve our services.

Q. The vast quantities of data produced by these surveys need to be stored and utilized. How do you handle this?

A. In the wireline work that Schlumberger carries out, the work of systems and software is always implied as being an essential part of our work. For heavy oil work, as indeed with all the work we do, we tailor and manipulate the data accordingly using our high-end systems.

For example, when we examine an heavy oil reservoir, we are looking at the type of rock, its mineral constituents, the different types of water bound to these minerals, waters held in the pore space, waters that move, waters that do not move and then the hydrocarbons themselves. We have a number of programs that we can adapt for most environments that let us set up a “solver” that when fed by the outputs from our various logging tools, allows us to quantify each individual piece. As a result, the client obtains a total and accurate evaluation of his reservoir.

Our processing technology is at the industry vanguard. We feed data into our systems for ELAN (Elemental Analysis). The output from an ELAN can be fed into Schlumberger ECLIPSE* reservoir simulators enabling reservoir engineers to run simulations; what is required to make it flow, predict how much oil will flow, and what type of equipment will be required on the surface. It is quite a long data trail.

Q. Can you give a specific benefit to some of the measurements you take?

A. Reservoir saturation tools (RST) when run in cased holes are the workhorse in oil reservoir monitoring services and have particular relevance in heavy oil reservoirs such as those in California and Venezuela. Oil saturation is of key importance to operators, as it gives an indication of the suitability of thermal stimulation. For example, as a steam front moves towards a producing well, the remaining fluids are one of two types: oil that has been left behind as it is too heavy to shift, or a mixture of original formation water and steam water. Reservoir saturation tools evaluate the type of fluid left behind in the rock, thereby indicating where the steam fronts have broken through. This, therefore, eliminates that zone from any further steam stimulation work and can contribute to more effective future steam injection.

Dielectric measurement is very sensitive to the amount of water present in a reservoir but virtually blind to the presence of oil. So in a heavy oil reservoir we can quantify the amount of pore space occupied by water, so the remaining pore space will be taken up by oil. This sort of measurement has been in use in heavy oil reservoirs from California, to Venezuela, and Central Africa.

We have had good success using our magnetic resonance tool in two ways. As described earlier, for the heaviest oils, we combine the measured porosity of the magnetic resonance tool to the porosity from another tool, such as the bulk density tool, and determine the volume of heavy oil. For oils with a viscosity less than about 200cP, magnetic resonance can differentiate the various fluid fractions in the pore space, and provide an estimate of the oil viscosity. These techniques make it possible for the oil companies to plan the economic development of their fields because it all relies upon the OIP.

Finally, an example where heavy oil identification was helpful to a customer in the Middle East: On the flanks of this particular oil reservoir, the oil is heaviest and a tar-mat had occurred which acted as an impermeable barrier, sealing off the vertical flow of fluids. By using wireline logging tools to understand the location and nature of the tar-mat the customer was able to produce the lighter oils without running the risk of coning water up from the lower parts of the reservoir.