Page 150 - Origin and Prediction of Abnormal Formation Pressures

126 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.

TABLE 5-2
Parameters affecting practical information pressure evaluation techniques (after Fertl and Chilingarian, 1987,
table III, p. 29)

Geologic factors
Geologic age changes. Compaction effects on regional (basin edge versus basin center) and local scale, and
differential compaction across structures. Sand/shale ratio in clastic sediments. Lithology effects: pure
shales (soft, hard); limey and silty shales; bentonitic markers; gas-bearing ('shale gas'), organic-rich,
bituminous shales; types and amounts of clay minerals in shales (depending on depositional environment
and/or diagenesis). Heavy minerals (siderite, pyrite, mica, etc.). Drastic formation water salinity variations
in subsurface. High geothermal gradients. Stratigraphic and tectonic features (acting as overpressure
continuities or barriers), including unconformities, pinchouts, and faults; proximity to large salt masses,
mud volcanoes, geothermal 'hot' spots, etc. Steep, thin, overturned beds. Pore pressure gradients within
single thick shale interval.
Borehole environment
Borehole size, shape, and deviation. Shale alteration and hydration (exposure time of open hole to drilling
mud). Type of drilling mud (freshwater, saltwater, oil-base). Type and amount of weighting material (barite,
etc.) and/or lost circulation material (mica, etc.). Degree of 'gas cutting' in mud.
Drilling conditions
Hole size, shape, and deviation. Mud programs and mud hydraulics (circulation rate). Rotary speed. Bit type
(button, diamond, insert, etc.). Bit weight to bit diameter ratio. Bit wear (sharp, new bits versus dull, old
bits). Degree of overbalance. Floater ('heave' action) versus fixed on- or offshore rig.
Sample selection
Type and size of sample (avoid sand, cavings, recirculated shales). Sampling technique. Sampling
frequency. Analysis methods: for example, cutting density-variable density column, multiple-density
solution technique (float and sink method), mercury pump technique, mud balance technique. Proper
calibration is of utmost importance.
Geophysical well logging
Different basic measuring principles (shales are anisotropic). Sonde spacing. Depth of tool investigations.
Temperature ratings. Proper tool calibration. Tool malfunction (overlapping repeats or returns).
Parameter plotting techniques
Interval (or sample) selection. Sampling frequency. Linear, logarithmic plots. Plot comparable data (not
compatible are bulk density from logs versus cuttings, and short normal versus induction log resistivities).
Proper selection of 'normal' compaction trendlines (discrepancies become enhanced with increasing depth
of wells). Use all information available. Experienced, properly trained personnel is a must.

PREDICTION OF ABNORMALLY HIGH PRESSURE IN REGIONS WITH NONEQUILIBRIUM
COMPACTION

Dobrynin and Serebryakov (1978) studied present-day pressures of formations in the
West-Kuban Depression, South Caspian Basin, Fergana Basin and other petroliferous
regions of the former USSR. They also estimated (theoretical) the possible lifetime
(duration) of abnormally high pressure in oil and gas accumulations.
The present-day values of pore pressure in the shale seals (caprocks) and the forma-
tion pressure in associated reservoir rocks in regions with nonequilibrium compaction
depends on (1) the permeability of the seals, (2) the lithology of the geologic section, (3)