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Drilling 143

27.0
24.0

21.0
Dolomite
Penetration rate (m/hr) 15.0 Marble
18.0

12.0

9.0
6.0 Sandstone

3.0 Shale

0.0
0 10 20 30 40 50 60
Pressure (MPa)
FIGUre 8.5 Penetration rate as a function of the confining pressure. (The curves are based on data and
fits from Black, A. and Judzis. A., 2005. Optimization of Deep Drilling Performance: Development and
Benchmark Testing of Advanced Diamond Product Drill Bits & HP/HT Fluids to Significantly Improve
Rates of Penetration. Topical Report, DE-FC26-02NT41657, http://www.osti.gov/bridge/product.biblio.
jsp?query_id=1&page=0&osti_id=895493; Lyons, K. D., Honeygan, S., and Mroz. T., NETL Extreme Drilling
Laboratory Studies High Pressure High Temperature Drilling Phenomena. National Energy Technology
Laboratory Report NETL/DOE-TR-2007–163, 1–6, 2007.)

heterogeneous nature of rocks. The impact of this heterogeneity on the bit performance can be
appreciated by considering a rock in which there are subvertical zones that are weakened due to
alteration or fracturing. When a bit first encounters such a region, the rock strength on one side of
the hole being drilled will be less than that of the rest of the hole. If the difference in strength is
significant and persists for some distance, the bit will wander into that region that is more readily
excavated. The result will be a well that changes alignment with depth. The magnitude of this effect
can be quite large, amounting to many meters or tens of meters of deviation from the planned ori-
entation. Changes in drilling rate, drill string abrasion, and other indicators provide the driller with
hints as to whether such effects are occurring. In severe cases, downhole measurements are used to
determine the actual orientation of the hole with depth. However, these are costly processes when
the rig is forced to shut down the drilling operation for some time period. It is for this reason—
among others—that care is taken when determining how to load the bit, since overweighting it can
encourage deviation.

TemperaTure and drillinG fluid sTabiliTy
Although standard drilling muds that are commercially available are adequate for many drilling
applications, as temperatures of 200°C are exceeded, the performance of standard drilling muds
begins to be compromised. The issues that are encountered include loss of plasticity, flocculation
(which affects the viscosity), and shrinkage. The reasons for these effects reflect the thermal and
chemical stability of clay minerals.
Consider the reaction for montmorillonite dehydration (refer back to Equation 8.1). It is apparent
that this reaction will be very sensitive to temperature, since the right-hand side of the reaction has
the high entropy compound, water, as a product. Furthermore, since there are multiple water mole-
cules in the structure of montmorillonite, multiple dehydration steps will affect the montmorillonite