Page 155 - Hydrocarbon

142 Data Gathering

reservoir parameters from a number of measurements taken in a borehole. The main
techniques available within these categories are summarised in the following table:

Direct Indirect

Coring Wireline logging
Sidewall sampling (SWS) Logging while drilling (LWD)
Mudlogging Seismic
Formation pressure sampling
Fluid sampling

This section will look at formation and fluid data gathering before significant
amounts of fluid have been produced, hence describing how the static reservoir is
sampled. Data gathered prior to production provide vital information, used to predict
reservoir behaviour under dynamic conditions. Without these baseline data no
meaningful reservoir simulation can be carried out. The other major benefit of data
gathered at initial reservoir conditions is that pressure and fluid distribution are in
equilibrium; this is usually not the case once production commences. Data gathered
at initial conditions are therefore not complicated by any pressure disturbance or fluid
redistribution, and offer a unique opportunity to describe the condition prior to
production.

6.3.2. Coring and core analysis
To gain an understanding of the composition of the reservoir rock, inter-reservoir
seals and the reservoir pore system, it is desirable to obtain an undisturbed and
continuous reservoir core sample. Cores are also used to establish physical rock
properties by direct measurements in a laboratory. They allow description of the
depositional environment, sedimentary features and the diagenetic history of the
sequence.
In the pre-development stage, core samples can be used to test the compatibility
of injection fluids with the formation, to predict borehole stability under various
drilling conditions and to establish the probability of formation failure and sand
production.
Coring is performed in between drilling operations. Once the formation for
which a core is required has been identified on the mud log, the drilling assembly is
pulled out of hole. For coring operations, a special assembly is run on drill pipe
comprising a core bit and a core barrel (Figure 6.32).
Unlike a normal drill bit which breaks down the formation into small cuttings, a
core bit can be visualised as a hollow cylinder with an arrangement of cutters on the
outside. These cut a circular groove into the formation. Inside the groove remains
an intact cylinder of rock which moves into the inner core barrel as the coring
process progresses. Eventually, the core is cut free (broken) and prevented from
falling out of the barrel whilst being brought to surface by an arrangement of steel
fingers or ‘catchers’. Core diameters vary typically from 3 to 7 in. and are usually