Page 164 - Fundamentals of Gas Shale Reservoirs

144 PORE PRESSuRE PREdIcTIOn FOR ShAlE FORmATIOnS uSInG WEll lOG dATA

(dickinson, 1951). The importance of the unloading mecha- 7.2.4.3 Kicks Kicks are also considered as overpressure
nisms such as gas generation and clay diagenesis processes has indications when occurring in balanced drilling operations.
been discussed by many authors. Example of basins where The advanced drilling technology may require the drillers to
overpressure was generated by unloading mechanism is the adjust the mud weight to a narrow line between effective
north Sea (hermanrud et al., 1998). The lateral stress could pressure control and the blowout, and this may cause sudden
also play an important role for generating overpressure in kick when penetrating overpressured formations. Thus, inap-
relatively old sedimentary basin such as the cooper Basin in propriate pressure balance between mud weights and formation
South Australia (Van Ruth et al., 2003). pore pressures may cause kicks (Fertl and Timko, 1971).

7.2.4.4 Mud Weight Kicks may be best avoided by reg-
7.2.4 Overpressure Indicators from Drilling Data
ular checks of the drilling mud properties, particularly for
The best technique to detect and assess overpressure is to any drop in the mud weight, and this can be used as an
study and combine all available pore pressure‐relevant indicator of gas cuttings and kicks.
parameters. depending only on one specific type of data can
lead to erroneous interpretations. In conjunction with well 7.2.4.5 Flow Line Temperature Temperature measurement
log data analysis, the mud log data at the surface can also be of the drilling flow line is a useful way of identifying changes
used as an indicator for penetrating overpressured forma- in temperature while drilling petroleum wells. Several
tions (Fertl and Timko, 1971). These measurements are authors such as lewis and Rose (1970) reviewed the heat
discussed in detail in the following sections and include the conductivity and suggested that whenever encountering
drilling rate of penetration (ROP), gas show at the surface, overpressured formations, formation temperatures increase.
kicks, mud weight, and the flow line temperature. The observed changes in the flow line temperature is of the
order by 2–10°F above the normal temperature when
7.2.4.1 Drilling Rate of Penetration (ROP) The evalua- entering the overpressured environments. however, it is
tion of drilling performance parameters, specifically the ROP, is important to bear in mind that temperature increase can also
used to detect overpressure formations (Fertl and Timko, 1971). be due to changes in lithology and the presence of salt dome
The ROP is inversely proportional to the differential pressure at (Fertl and Timko, 1971).
the bottom hole between formation pressure and the hydrostatic
pressure which results from the mud weight column. The 7.2.5 Identification of Shale Intervals
advantage of using the ROP data over log data analysis for
overpressure detection is the immediate availability during the A typical definition of shale points to three main characteris-
drilling operations (Jorden and Shirley, 1966). tics: (1) the clay content forms the load‐bearing framework;
Field examples show the normal trend for the ROP (2) the pore size of shale, which is measured on the nanometer
reduces as depth increases with the other drilling parameters scale and permeability, which is measured by nanodarcy; and
remaining constant. under constant mud conditions while (3) the surface area of shale is large and adsorbs water easily.
drilling normally pressured formations, the bottom hole The basic data available for petrophysicists for shale
differential pressure increases as a result of the increase in identification are well log data. Shale pore pressure estimation
the formation pressure as depth increases: the effect is a from well logs is based on compaction theory, which requires
reduction in the ROP. By contrast, when drilling overpres- establishing normal compaction trends (ncTs) within contin-
sured sections under the same drilling conditions, the bottom uous shale and a uniform lithology section. In order to dis-
hole differential pressure decreases and the ROP increases criminate shale intervals from other lithology sections, a
thereafter (Fertl and Timko, 1971). Therefore, under constant gamma‐ray log that measures the rocks radioactivity is gener-
drilling conditions, it is believed that the ROP decreases ally used to measure the clay contents (Fig. 7.7).
while drilling normally pressured formations and deviates to It was suggested by Fertl (1979) that the use of gamma ray
a higher rate when encountering overpressured formations. for shale discrimination may cause errors as shale radioac-
tivity varies significantly from one shale to another and not all
7.2.4.2 Gas Show There are many possible sources of shales are radioactive. likewise, sandstone grains may have
gas show in the mud returned to the surface. According to radioactive materials. Based on petrophysical properties of
Fertl and Timko (1971), the origins of these gases could be rocks, Katahara (2008) proposed a technique for distinguish-
(i) underbalanced conditions when drilling overpressure for- ing shale from sandstone, which is based on the difference
mations, (ii) gas released from the cutting, and (iii) gas‐ between neutron porosity and density porosity. This approach
bearing rocks. The compounds of the returned gas can be is a more appropriate technique for measuring clay contents
studied, and certain components can be related to overpres- than using gamma ray, and it is a simplified model where
sure. hence, a sudden increase of connection gas and gas in quartz, clay minerals, and water are the only components of
the mud may be an indication of overpressured formations. shale (Fig. 7.8). The shale falls within the triangle constrained