A variety of techniques are employed in a laboratory environment to simulate the drilling of deep, underground formations. For example, rock (earth) samples may be prepared such that they can be subjected to stresses that simulate those that might be expected in the field environment. These rock samples may be surrounded by impermeable sleeves which are affixed to at least one end cap on at least one distal end of the rock sample. Two end caps may be held together by multiple tie rods positioned outside the impermeable sleeve that covers the rock sample. To simulate a subterranean drilling process, the rock sample is installed in a drilling apparatus that is capable of applying at least some of the horizontal, vertical, pore, drilling fluid, and wellbore stresses that might be encountered in subterranean drilling. A drill bit affixed to the end of a rotating shaft then engages the stressed rock sample and forms a wellbore as a consequence of a compressive load applied axially through the rotating shaft. When simulating the drilling of depleted formations, the pressure of the drilling fluid in the wellbore may be raised to a level exceeding the compressive axial stresses that hold the end cap(s) and the rock sample in contact with each other. When the wellbore pressure is raised to a level that results in a load that exceeds the externally applied load holding the end caps in contact with the rock sample, the end caps may separate from the rock surface, thus allowing the drilling fluid under pressure to flow across the interface and into the space between the external surface of the rock sample and the interior of the impermeable surrounding sleeve. Inflation of the surrounding sleeve can potentially rupture the sleeve. The fluid path between the wellbore and the external periphery of the rock sample detrimentally affects the stress distribution on the rock sample. In order to ensure that the end caps do not separate from the rock sample and a fluid path does not develop, a vertical stress is frequently applied to the sample that is in excess of what is desired for the experiment. The present disclosure describes a means for preventing drilling fluid from traversing the rock sample/end cap interface that is independent of the applied vertical stress.
USPTO Backgrounds
USPTO Backgrounds