GB1485853A - Methods and apparatus for testing earth formations composed of particles of various sizes - Google Patents

Abstract

1485853 Formation fluid sampling SCHLUMBERGER Ltd 17 Oct 1974 [18 Oct 1973 (2)] 45020/74 Heading E1F Formation fluid samples are obtained from a borehole 12 whose walls have a lining of mudcake by packing off a portion of the borehole wall to isolate that portion from fluids in the borehole and inducting formation fluids from the isolated portion of the borehole wall through a fluid entry and filtering means which initially allow the passage of particles of mudcake but which prevent the passage of loose particles eroded from the formation when unconsolidated formations are being tested. As described the formation testing tool 11 includes a tool body 19 which can be lowered down a borehole 12 by a cable 15 coupled to the above ground control system 16, recording apparatus 17 and power supply 18. When the body 19 reaches the formation to be tested, e.g. formation 13, hydraulic rams 21, 41 are operated to move tool anchor 20 and fluid admitting means 10 into engagement with formation 13. The fluid admitting means 10 (Fig. 2A) includes a sealing pad 39 and a tubular fluid-admitting member 42 which is urged forwardly into contact with the wall of the borehole by supplying pressure fluid to chamber 50, the pad 39 and member 42 isolating a portion of the borehole wall from the borehole fluids. The tubular member is initially closed by a piston head 53 which can be retracted by supplying pressure fluid to chamber 63. When the piston head 53 is retracted, formation fluids can flow into the tubular member 42, through a cylindrical filter 66 into annular space 64 and through passages 65, 57, 54, 55 into conduit 68 which is connected to one or more sample collecting chambers 22, 23 (Fig. 1). The pressure of the formation fluid may also be measured and recorded by apparatus 17. In a first embodiment (Figs. 2A, 8A) the filter 66 has narrow forward slots 144, intermediate slots 145 and wide rearmost slots 143. The slots 143 are wide enough to allow particles of mudcake from the borehole wall to pass through them and the forward slots 144 are small enough to prevent the passage of loose formation particles, e.g. sand particles. The result of this arrangement is that, when testing unconsolidated formations, the loose particles bridge over the slots 143 since a major portion of the fluid flow is through the slots 144 which prevent the passage of the particles, so that the end of tube 42 fills up with the particles and prevents further erosion of the formation. In a modification (Fig. 9A) the same result is achieved by a filter 66' having longitudinal slots which are wider at their rearmost ends 143' than at their forward ends 144'. In a second embodiment (Fig. 8B) the filter consists of two tubes 45', 46' having slots 141', 142' respectively. Tube 46' is fixed to tubular member 42 and tube 45' is movable between a position in which slots 141', 142' are in registry and a position in which the slots 141', 142' are in partial registry to provide smaller openings, the tube 45' being biassed to this latter position by spring washer 49'. When testing consolidated formations and when initially engaging unconsolidated formations the pressure exerted on the end of the tube 45' is sufficient to compress springs 49' so that the slots 141', 142' move into registry and allow the passage of particles of mudcake. However when testing unconsolidated formations, when tube 42 has penetrated the formation as shown in Fig. 8B the pressure on tube 45' is insufficient to compress springs 49' and the slots 141', 142' move into partial registry to prevent the passage of loose sand particles. In both embodiments loose sand particles in tube 42 are expelled by piston head 53 after the sample has been taken.