1. Field of the Invention
The present invention relates generally to the field of downhole sampling and in particular to the maintenance of hydrocarbon samples in a single-phase state after capture in a sample chamber.
2. Summary of the Related Art
Earth formation fluids in a hydrocarbon producing well typically comprise a mixture of oil, gas, and water. The pressure, temperature and volume of formation fluids control the phase relation of these constituents. In a subsurface formation, high well fluid pressures often entrain gas within the oil above the bubble point pressure. When the pressure is reduced, the entrained or dissolved gaseous compounds separate from the liquid phase sample. The accurate measure of pressure, temperature, and formation fluid composition from a particular well affects the commercial interest in producing fluids available from the well. The data also provides information regarding procedures for maximizing the completion and production of the respective hydrocarbon reservoir.
Certain techniques analyze the well fluids downhole in the well bore. U.S. Pat. No. 6,467,544 to Brown, et al. describes a sample chamber having a slidably disposed piston to define a sample cavity on one side of the piston and a buffer cavity on the other side of the piston. U.S. Pat. No. 5,361,839 to Griffith et al. (1993) disclosed a transducer for generating an output representative of fluid sample characteristics downhole in a wellbore. U.S. Pat. No. 5,329,811 to Schultz et al. (1994) disclosed an apparatus and method for assessing pressure and volume data for a downhole well fluid sample.
Other techniques capture a well fluid sample for retrieval to the surface. U.S. Pat. No. 4,5 83,595 to Czenichow et al. (1986) disclosed a piston actuated mechanism for capturing a well fluid sample. U.S. Pat. No. 4,721,157 to Berzin (1988) disclosed a shifting valve sleeve for capturing a well fluid sample in a chamber. U.S. Pat. No. 4,766,955 to Petermann (1988) disclosed a piston engaged with a control valve for capturing a well fluid sample, and U.S. Pat. No. 4,903,765 to Zunkel (1990) disclosed a time delayed well fluid sampler. U.S. Pat. No. 5,009,100 to Gruber et al. (1991) disclosed a wireline sampler for collecting a well fluid sample from a selected wellbore depth, U.S. Pat. No. 5,240,072 to Schultz et al. (1993) disclosed a multiple sample annulus pressure responsive sampler for permitting well fluid sample collection at different time and depth intervals, and U.S. Pat. No. 5,322,120 to Be et al. (1994) disclosed an electrically actuated hydraulic system for collecting well fluid samples deep in a wellbore.
Temperatures downhole in a deep wellbore often exceed 300 degrees F. When a hot formation fluid sample is retrieved to the surface at 70 degrees F., the resulting drop in temperature causes the formation fluid sample to contract. If the volume of the sample is unchanged, such contraction substantially reduces the sample pressure. A pressure drop changes in the situ formation fluid parameters, and can permit phase separation between liquids and gases entrained within the formation fluid sample. Phase separation significantly changes the formation fluid characteristics, and reduces the ability to evaluate the actual properties of the formation fluid.
To overcome this limitation, various techniques have been developed to maintain pressure of the formation fluid sample. U.S. Pat. No. 5,337,822 to Massie et al. (1994) pressurized a formation fluid sample with a hydraulically driven piston powered by a high-pressure gas. Similarly, U.S. Pat. No. 5,662,166 to Shammai (1997) used a pressurized gas to charge the formation fluid sample. U.S. Pat. No. 5,303,775 (1994) and U.S. Pat. No. 5,377,755 (1995) to Michaels et al. disclosed a bi-directional, positive displacement pump for increasing the formation fluid sample pressure above the bubble point so that subsequent cooling did not reduce the fluid pressure below the bubble point.
Existing techniques for maintaining the sample formation pressure are limited by many factors. Pretension or compression springs are not suitable because the required compression forces require extremely large springs. Shear mechanisms are inflexible and do not easily permit multiple sample gathering at different locations within the well bore. Gas charges can lead to explosive decompression of seals and sample contamination. Gas pressurization systems require complicated systems including tanks, valves and regulators which are expensive, occupy space in the narrow confines of a well bore, and require maintenance and repair. Electrical or hydraulic pumps require surface control and have similar limitations.
Accordingly, there is a need for an improved system capable of compensating for hydrostatic well bore pressure loss so that a formation fluid sample can be retrieved to the well surface at substantially the original formation pressure, that is, in a single phase state. The system should be reliable and should be capable of collecting the samples from the different locations within a well bore.
Unlike an ordinary sample tank, however, a single-phase tank has a floating piston inside of it. Sample fluid or crude is pumped into the sample tank against the top side of the piston. Downhole, as crude oil is pumped into the tank, the pumped crude pushes against the top side of the floating piston inside of the sample tank and further compresses the gas cushion underneath the sample tank piston. Crude oil is pumped into the sample tank against the cushioned piston until its pressure is several thousand pounds per square inch above formation pressure. The gas cushion is initially created at the surface where the tank is charged before going into the well bore. The purpose of charging the down hole sample tank is to maintain the down hole sample of crude oil in a single phase condition after it has been brought to the surface and cools. Gas is pumped underneath the sample tank piston to charge the sample tank cylinder.
To charge the single-phase sample tank cylinder a non-reactive gas (e.g., nitrogen) is connected to the sample tank through a pressure regulator. The tank is filled until the pressure underneath the sample tank piston reaches the set pressure of the regulator. The tank inlet valve is then closed thereby trapping as many moles of gas as can possibly fit into the tank volume underneath the piston at that pressure. This gas cushion is important when collecting down samples of crude oil at elevated temperatures of 100-200 C and pressures of 10-20 kpsi. As these tanks are brought back to the surface, the tank and the sample inside of the tank, once removed from the high temperature down hole in the well bore, cools to the ambient surface temperature so the crude oil within the sample tank shrinks or reduces its volume and pressure associated therewith is likewise reduced. This temperature-induced shrinkage can be as much as 30% of the initial crude oil volume. At this reduction in pressure, below the bubble point for the crude, it is expected that natural gas bubbles will nucleate or asphaltenes precipitate and come out of the crude oil and fill the void left by shrinking liquid. Nucleation of gas bubbles or precipitation of solids changes the single-phase liquid crude to a two-phase state consisting of liquid and gas or liquid and solids. Two-phase samples are undesirable, because once the crude oil sample has separated into two phases, it can be difficult or impossible and take a long time (weeks), if ever, to return the sample to its initial single-phase liquid state even after reheating and/or shaking the sample to induce returning it to a single-phase state.
Due to the uncertainty of the restoration process, any pressure-volume-temperature (PVT) lab analyses that are performed on the restored sing-phase crude oil are often suspect. When using ordinary sample tanks, one tries to minimize this problem of cooling and separating into two-phase by pressurizing the sample down hole to a pressure that is far (4500 or more psi) above the downhole formation pressure. The extra pressurization is an attempt to squeeze enough extra crude oil into the fixed volume of the tank that upon cooling to surface temperatures the crude oil is still under enough pressure to maintain a single-phase state and maintains at least at the pressure that it had downhole.
The gas cushion of the single-phase tanks, thus, makes it easier to maintain a sample in a single phase state because, as the crude oil sample shrinks, the gas cushion expands to keep pressure on the crude. However, if the crude oil shrinks too much, the gas cushion (which expands by as much as the crude shrinks) may expand to the point that the pressure applied by the gas cushion to the crude falls below formation pressure and allows asphaltenes in the crude oil to precipitate out or gas bubbles to form. Thus, there is a need for a gas cushion pressurization tank that maintains the single-phase state of a sample without requiring inordinately large and possibly dangerous pressures to be used in charging a sample tank before going down hole.