Page 164 - Hydrocarbon Exploration and Production Second Edition

Reservoir Description 151

be required. The quantity of data that can be transmitted in real time is limited and
focused towards responses used for geosteering and other drilling decisions. A full
data set is stored in the downhole memory which is retrieved when the tool is
brought to the surface.
Electrical power is supplied to LWD tools either from batteries run in the
downhole assembly or from an alternator coupled to a turbine set in the mudstream.
In terms of log data quality and tool response modern LWD/FEWD tools can be
as good as their wireline counterparts. However, the biggest issue when comparing
the two technologies is depth control. Wireline depth is accurately measured at the
surface by a spooling wheel in front of the cable drum which records the length of
cable that has been reeled in. LWD depth is measured as ‘drillers depth’ where the
driller records the length of drill pipe that has been run in the hole. Individual
lengths of pipe can differ and the ‘pipe tally’ (record of pipe length) is not always
accurate. Additionally, the actual length of pipe in a long borehole may change
depending upon the amount of compression or tension within the string. On
occasions when LWD and wireline logs over the same section are compared, the
depth differences can be up to several metres. Currently, LWD companies are
designing depth-adjustment software to overcome this problem.

6.3.6. Pressure measurements and fluid sampling

A common objective of a data gathering programme is the acquisition of fluid
samples. The detailed composition of oil, gas and water is to some degree required
by almost every discipline involved in field development and production.
One method of sampling reservoir fluids and taking formation pressures under
reservoir conditions in openhole is by using a wireline FPT. A number of wireline
logging companies provide such a tool under the names such as RFT (repeat
formation tester) and FMT (formation multitester), so called because they can take a
series of pressure samples in the same logging run. Newer versions of the tool are
called a modular dynamic tester or MDT (Schlumberger tool), shown in Figure 6.39
and reservoir characterisation instrument or RCI. The latest LWD/FEWD technology
includes FPTs.
The tool is positioned across the objective formation and set against the side of
the borehole by either two packers or by up to three probes (the configuration used
will depend on the test requirements). The probes are pushed through the mudcake
and against the formation. A pressure drawdown can now be created at one probe
and the drawdown be observed in the two observation probes. This will enable an
estimate of vertical and horizontal permeability and hence indicate reservoir
heterogeneities as well as recording a pore pressure. Alternatively fluids can be
sampled. In this case, a built-in resistivity tool will determine when uninvaded
formation fluid (hydrocarbons or formation water) is entering the sample module.
The flow can be diverted back into the wellbore until only the desired fluid is
flowing, thus providing fluid samples uncontaminated with mud. The pressure
drawdown can be controlled from surface, enhancing the chance of creating a
monophasic flow by keeping the drawdown above bubble point.