Page 258 - Drilling Technology in Nontechnical Language

Chapter 10 – EVALUATION 249

Logging while drilling (LWD)

In the last 20 years or so, logging sensors have been built into drill collars
that are robust enough to withstand the heat, shock loadings, and vibration
of drilling. These tools record log data while drilling progresses.
Measurement while drilling (MWD) tools were described in chapter
8. If log data is required in real time at the surface, the telemetry system
of an MWD tool is used to transmit the data to the surface using mud
pressure pulses. This requires that both an LWD tool and an MWD tool
are included in the BHA, and a physical connection using electrical wiring
is made between the two tools. In a deviated well, it may also be necessary
to transmit MWD data so that the bit can be accurately navigated to
the target.
LWD tools can also be run in record mode, in which data is recorded
within a memory module inside the tool. When the tool returns to the
surface, this data is downloaded to a computer for analysis and printing. If
the telemetry link fails to provide real-time data, the memory should still
provide the log data.
LWD tools have developed to the point where the data they provide is
accurate and reliable enough to replace many wireline logging tools. The
cost of using LWD is then partially offset by the time and cost saving of
avoiding wireline logging. LWD can also obtain data in conditions where
wireline tools cannot be run. If navigating within the reservoir (say in
a horizontal well), the LWD data becomes important to ensure that the
wellbore stays in the correct place.

Production Testing

While evaluation of cuttings, mud, drilling parameters, and electrical
logs is vital to understand the static reservoir characteristics, the only way
to ascertain dynamic reservoir performance is to let the well flow. An

exploration well that locates possibly commercial quantities of hydrocarbons

is usually tested by flowing it for periods of time and measuring the
response (pressure) of the reservoir. This allows the operator to build a
model to predict production rates and total hydrocarbon volumes for the
reservoir under different operating conditions. The produced hydrocarbons

are burned, which is hot, noisy, and often spectacular (fig. 10–11).

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