Stabilizing Sensitive Shales With Inhibited, Potassium-Based Drilling Fluids
Stabilizing Sensitive Shales With Inhibited, Potassium-Based Drilling Fluids
Stabilizing Sensitive Shales With Inhibited, Potassium-Based Drilling Fluids
Introduction
Proper selection of the drilling fluids to be used on considerations, however, suggest that water- based
a particular wellsite is an essential phase of any care- fluids be designed to perform the same function.
fully planned drilling operation. When this drilling is In recent years many new water-based mud sys-
expected to encounter water-sensitive shale zones, tems have been ·devised. 2, 3 Most of these rely on
the selection of the fluid becomes even more impor- polymer or combinations of polymer and salt to
tant. To maintain a stable borehole through such achieve a given level of inhibition. Low-solids drill-
zones, an inhibitive drilling mud will often be re- ing, using total or selective polymeric flocculants to
quired. The design of successful fluids for this type achieve controlled clay yields, has been an important
of application depends largely on a knowledge of step forward. The new directions that drilling fluids
the physical and mineralogical characteristics of the in general and inhibitive fluids in particular are taking
shale and its behavior when contacted by drilling promise still further advances.
mud. We shall discuss here procedures and experi-
mental results of laboratory tests in which a wide Causes of Shale Problems
variety of fluids and shale types were used. The data To investigate properly the nature of problems in
should aid in designing effective inhibitive muds. drilling shales one must first study the various related
Sensitive shale fornlations are encountered in most physico-chemical processes. The general class of
major oil fields throughout the world. These forma- shale problems presented in the introduction results
tions, referred to by a variety of names, including from the following interrelated factors: (1) shale
mud-making shale, caving shale, heaving shale, and hydration and swelling, (2) dispersion of shale cut-
sloughing shale, frequently cause wellbore instabili- tings, and (3) abnormal pressure.
ties. Drilling and completion problems associated In addition, other processes and borehole condi-
with such instabilities include the accumulation of tions tend to aggravate the instability. These include
fill on the bottom after trips, tight holes, stuck drill (1) time spent in the open shale zone, (2) in-situ
pipes, solids buildup in the mud, hole enlargement stresses and formation characteristics, and (3) me-
and washouts, and poor primary cement jobs. Stable, chanical and erosive action.
gauge boreholes, reduced rig time, and lower total
costs could be realized if efficient inhibitive systems Shale Hydration and Swelling
were employed. Presently the only way to insure Shales are basically sedimentary rocks that have been
success in many problem formations is to use laid down over geologic time in marine basins. They
balanced-activity oil muds. 1 Cost factors and handling are usually composed of quartz, feldspar, calcite, and
Laboratory and field studies have demonstrated that potassium cations at sufficien,t
concentrations in water-based drilling fluids can effectively reduce the swelling and
dispersive tendencies of clay-containing shales. Here is a unified classification method
for relating the amount of water-sensitive clay in a shale to the occurrence and severity
of wellbore problems.
-3700-
6400 8400 >
CALIPER
-4000-
6500 8500
-4300-
6600 8600
-4600-
~
-4900-
6700 ~ 8700
6800 8800
-5300-
7000 9000
TEST WELL OFFSET WELL
KCl XC POLYMER MUD 1550'-7300' GEL-LIGNITE MUD 1600'-7000'
7" 8" 9" 10" n" 12" 7 11 8" 9" 10" n/l 12" 7100 9100
o >
7200 9200
1000
7300 9300
2000
7400 9400
3000
7500 9500
4000
7600 9600
5000
7700 9700