Development and Field Testing of a Core Barrel for Recovering Unconsolidated Oil Sands

Abstract

The rubber-sleeve core barrel was developed to improve core recovery from unconsolidated sands, where it is most difficult to obtain cores with conventional barrels. The use of a rubber-sleeve core retainer, together with other departures from usual design, has required a considerable amount of testing to prove its operability. Tests run in shallow experimental holes and in drilling wells in the U.S. and Venezuela have, to a large extent, proved the operation of the barrel. Internal mechanism troubles, such as sandbinding of the sleeve, have been largely overcome. Cores have been recovered from completely unconsolidated sands where conventional coring has almost been abandoned because of poor recovery. In addition to retaining the core until brought to the surface, the barrel gives a "packaged-as-cut" core, which is convenient for handling and transportation. In the initial work, drag blade cutters, roller cutters, and tungsten carbide insert cutters were used for cutting unconsolidated sands. Further bit head development is presently being conducted using diamond heads in unconsolidated sands where shales, hard sands, and limestones are encountered in adjacent formations. This work should render the barrel capable of cutting and recovering most types of formations, possibly including broken and fractured formations. This development is being carried on in cooperation with a diamond bit manufacturer. Introduction For many years the oil industry has needed greater recovery of cores from unconsolidated formations. Conventional equipment has been used to core unconsolidated sands with little success. Specifically, a core barrel was needed which would recover unconsolidated sands in a condition that would give useful reservoir information. Development of the Rubber-Sleeve Core Barrel Basic Requirements of Unconsolidated Sand Coring The basic need was defined as a core barrel that would obtain a continuous unconsolidated core with sand grains in place as deposited in the formation. A preliminary survey of coring equipment and search of the literature pointed out the weaknesses of conventional core barrels. Laboratory tests using unconsolidated sand in a metal tube simulated actual sand movement into the care barrel during conventional coring. In conventional barrels the core sometimes crumbles and bridges, or fails under compressive column action, becomes oversize, and wedges against the walls of the inner barrel.