1. Field of the Invention
The present invention pertains to apparatus and methods for obtaining core samples from subterranean formations. More specifically, it pertains to apparatus and methods for obtaining core samples from the bottom of a well bore, primarily in producing and exploring for oil and gas or other minerals. In particular, the present invention is directed to apparatus and methods for obtaining pressurized core samples and raising them to the surface of the well without losing liquids and gases contained therein and without creating an overpressured and potentially dangerous core barrel.
2. Brief Description of the Prior Art
It has long been a practice in drilling of oil and gas wells to monitor or examine the subterranean formations in which the well is being drilled by obtaining a core sample at the bottom of the well bore. Customarily, a core is obtained by drilling an annulus at the bottom of the well bore, leaving a cylindrical core section extending upwardly within a core barrel of some type. The core section or sample is then broken off and retrieved to the surface for examination and analysis. In the past, such coring was done by cable tool devices or by removing the drill string with the core sample captured therein. The core sample is of course initially at the bottom of the well where it may be under very high formation pressures. In core devices of the prior art, the natural pressure within the core is reduced as the core is brought to the surface until it is essentially at atmospheric pressure. As this pressure is reduced, it is probable that much of the liquids and gas, which are naturally captured in the core, will be released so that once the core reaches the surface, it is difficult to determine what may have been contained in the strata at the bottom of the hole.
Because of the problems associated with losing formation pressure, core barrel devices have been developed which capture the core at the bottom of the well, then seal it within a pressurized barrel for removal to the surface. One such cable device is shown in U.S. Pat. No. 2,347,726. Other pressure capturing core barrel devices for attachment to the lower end of a drill string are shown in U.S. Pat. Nos. 3,064,742; 3,146,837; and 3,454,117. However, since the pressure at the bottom of a well hole may be extremely high, e.g. 10,000 p.s.i., the core in such an assembly may be encapsulated in a cylinder which is potentially a bomb when it reaches the surface. It may be hazardous to disassemble and requires extremely careful handling at the surface with specialized equipment.
In recent years, one or two core sampling devices have been developed which totally capture a pressurized sample, but allows for some reduction of pressure within the core barrel as it is raised to the surface so that when the core sample reaches the surface, it is at a much more practical pressure for working with, yet retains the liquids and gases present in the sampled strata. One such device is disclosed in U.S. Pat. No. 2,287,909 (Sewell). In Sewell, a rubber sleeve, silfon bellows, rubber baloon or other type of expansion member is provided in fluid communication with the chamber in which the core sample is received. However, the type of expansion members disclosed in Sewell are of relatively small volume and are not suitable for the high pressure formations encountered in modern drilling practice. It is not uncommon to encounter formations in which gas or vapors therein will expand several hundred times at atmospheric pressure. The coring device of Sewell could not accommodate such expansion and would probably fail well before reaching the surface.
While othbrs may have attempted to develop coring apparatus suitable for capturing high pressure core samples for sealed retrieval to the surface, while allowing the pressure within the device to be reduced as it is brought to the surface of the well. Since such sampling apparatus and methods are highly desired for proper sampling and analysis, the search will undoubtedly continue to overcome the disadvantages of coring apparatus and methods of the prior art.