|BRITAIN'S OFFSHORE OIL & GAS
Getting the Most out of a Well
Vital information on the type of rock drilled and the fluids it contains often needs to be obtained either while actually drilling, or after drilling before running casing (F74). This is obtained by running electronic measuring devices into the well - either while drilling (as part of the drillstring) or after drilling on "wireline" (F73). The various types of measurement include: (1) electrical resistivity of fluids within the rock; (2) the speed of sound through the rock; (3) reaction of the rock to gamma ray bombardment; (4) production of gamma rays from fluids within the rock due to neutron bombardment; and (5) natural gamma radiation of the rocks. The data obtained give indications of rock type and porosity and the presence of oil or gas.
Other devices measure hole diameter, dip of strata and the direction of the hole. Sidewall corers which punch or drill out small cores of rock, geophones for well velocity surveys and seismic profiling are also lowered into uncased wells. In deviated wells approaching the horizontal, flexible high-pressure steel coiled tubing may be used to carry wireline logging tools and for performing wellbore maintenance operations.
If oil or gas has been detected in a well, a tool is lowered on a wireline to measure fluid pressures and collect small samples. If the flow rate of the well needs to be measured, a "well test" is carried out. This involves running production tubing with flow control valves and isolation packers into the well, then flowing the hydrocarbons to surface through the high pressure pipework containing pressure recorders and flowmeters.