How much oil will come out of an oil field? Is it worth developing once it is discovered? These are not trivial questions if you must drill and complete your wells in deep water offshore or on the remote North Slope of Alaska. All of the major oil companies maintain core testing labs whose job it is to evaluate the oil producing potential of a given field based on the evaluation of oil cores. An oil core is a sample of the oil-bearing rock as obtained from exploratory drilling. The idea is to subject the recovered core to down-hole temperatures and pressures, then measure the flow of fluids through it. Oil producing rock typically has the density and porosity of cement, so the study of fluid flow in these materials requires the ability to measure small differential pressures (just a few psi) at very high static pressures (several thousand psi).
The oil core is prepared by fitting it into a special jacket that is heated to down-hole temperature. A special high-pressure pump forces brine through the core. The static pressures around the core are typically 5000 to 10,000 psig.
The core has sealing packers placed along its length at regular intervals. A Validyne variable reluctance pressure transducer like the P365 is plumbed between the packed-off sections so that the pressure drop through the core rock as a function of flow rate can be measured. Validyne offers several models of transducer that have a full scale of little as 5 psi of differential pressure, while both ports of the transducer are at a static pressure of 10,000 psig. A carrier demodulator displays the differential pressure digitally in engineering units. The relationship between flow and pressure drop is a measure of the permeability of the oil producing formation, and this can be used to determine the amount of oil that can ultimately be brought into the well bore from the surrounding rock.
When an oil field is first drilled and brought into production something like 85% of the oil typically never gets to the well bore because of the low porosity and permeability of the reservoir rock. Secondary recovery methods are employed to enhance oil production from existing wells to extend the economic life of the oil field. This often involves injecting natural gas back into the rock formation to build the pressure that will drive the oil to the well bore, and to sweep unrecovered oil from the tiny pores inside the rock.
There is much petroleum research devoted to this subject including a theoretical effort to describe those secondary techniques that are most effective. The problem is that reservoir rock is created by nature and all differ in some degree from the ideal. One way to research secondary recovery on a theoretical level is to create small samples of simulated reservoir rocks using semiconductor micro-machining techniques. In this way a thin slice of ‘rock’ can be created that has a theoretically ideal geometry and porosity. In this way secondary recovery techniques can be tested and observed on a micro level.
Simulating Ideal Reservoir Rocks:
A typical set up is shown below. Small squares are machined into a this sample to simulate the reservoir rock. Natural gas and various surfactants are pumped as a foam through the sample and the process recorded by video and pressure measurement. At one laboratory the Validyne P61 USB Pressure Transducer was used to measure the pressure drop across the sample as the foam is introduced. The idea is that by using a foam instead of just natural gas, the pressures and sweeping efficiency will be increased. This can be tested on a micro scale for an ideal slice of rock, and the results extrapolated to actual field conditions.
The P61 USB Pressure Transducer can measure low pressures effectively and connects directly to the PC data acquisition system via a USB port. The virtual serial COM port for the P61 is easy to integrate into data acquisition software, in this case MATLAB. The pressure drop across the sample is one indication of how effective the foam will be when used to increase oil production by secondary techniques.