As with everything oil and gas related, the amount of time it takes to extract the hydrocarbons from the subsurface geology increases the cost of the well. Because oil and gas operators are at the mercy of the commodities market, this increased cost reduces the profit margin of the overall project.
Determining the properties of the oil and gas mixture in the reservoir is vital to understanding how easily the hydrocarbons are going to flow from the well in its current state and allows the operator and its geologists to select the most cost-effective extraction methods.
Defining Pressure Volume Temperature (PVT Analysis)
PVT analysis can be done at just about any stage in the hydrocarbon extraction process, but obviously the greatest gains
“…for a given fluid and pore size with a constant wetting, the pressure required to force an air bubble through the pore is inversely proportional to the size of the hole.”
To understand the drive mechanics of the fluids in an oil well, a test is performed to determine the bubble point of the sample fluid. This test and the subsequent understanding of the bubble test is a crucial component to understanding the characteristics of the reservoir.
A bubble test communicates important information to the geologist about the expected recovery of the oil in a well. If the oil that will be recovered is undersaturated (containing very little dissolved gas), only a small percentage of the oil is recoverable without the use of some advanced recovery method.
Flash liberation is a process involving a sudden pressure change that causes the crude oil to quickly separate or “flash” into in an oil and gas mixture.
In the differential liberation process, the gas that is liberated from the liquid solution is continuously removed from the test sample, as opposed to the flash liberation process that allows the sample solution to reach equilibrium.
Reservoir to Surface Volume Relations
Determining the bubble point is only one step involved in a thorough PVT analysis. Flash and differential liberation tests yield three other important factors needed to determine the relationship between reservoir to surface volume – oil formation volume factor (Bo), gas formation volume factor (Bg) and solution gas-oil ration (Rs).
Oil Formation Volume Factor (Bo)
Bο is the ratio of the volume of oil at reservoir conditions to that of surface conditions. It is used to convert the flow rate of oil at the surface to that of reservoir conditions.
Gas Formation Volume Factor (Bg)
Bg is the ratio of the volume of gas at the reservoir temperature and pressure to the volume at the surface temperature and pressure.
Solution Gas-Oil Ratio (Rs)
Rs is the amount of gas dissolved in the oil (or water) at any pressure. It increases approximately linearly with pressure, and it is a function of the oil (or water) and gas composition. A heavy oil contains less dissolved gas than a light oil.
Compositional Analysis: The Real Goal of PVT Analysis
The reason any oil and gas company decides to drill a well is to turn the project into an oil-producing asset. But the value of the oil extracted from a single well is not the same as the value of the oil produced from another.
The makeup of the oil, which can be determined from the compositional analysis, is an important piece of the equation that determines how profitable the play will be.
The compositional analysis will determine just how much of each type of petroleum product can be produced from a single barrel of oil from that well – how much gasoline, kerosene, diesel, etc.
Each refinery is also set up to process only certain types of crude oil. A light crude and a heavy crude have very different processes, and therefore require different methods of refining. While this may seem like a small logistical concern, the difference in the composition of the crude oil a well produces could be the difference in refining along the U.S. Gulf Coast where refineries like heavy crude, or sending the oil to the East Coast of the United States or Europe, where the refineries prefer light sweet crude.
Sierra Pine Resources International has decades of experience in all types of upstream oil and gas consulting. We have several case studies with varying types of geology available for free download on our Case Studies page. Or you can contact one of our consultants directly.