CHAPTER-2
PREDICTING FORMATION PRESSURES
Several methods of pressure prediction is available to the engineer. The methods can be grouped logically as follows:
full hou-areal analysis from ismic data,
-off-t well correlation log analysis, drilling parameter evaluation, production or test data,
-real time evaluation as , a) qualitative, b) quantitative.
The real time analysis involves monitoring drilling and logging parameters while the prospect well is drilled.
Origin of Abnormal Pressures
By definition, abnormal pressure is any geo-pressure that is different from the established normal trend for the given area and depth. Pressure may be less than normal, called sub-normal or greater than normal pressure which has been termed geo-pressured, super pressured or simply abnormal pressure.
Sub-normal pressures prent few direct well control problems. However, subnormal pressures do cau many drilling and well planning problems. For clarity, the term abnormal pressure will identify the pressures greater than normal. Formation pressure is the prence of fluids in the pore spaces of the rock matrix. The fluids are typically gas or salt water. The overburden stress
is created by the weight of the overlying rock matrix and the fluid filled pores. The rock matrix stress is the overburden stress minus the formation pressure. For general calculations the overburden stress gradient is often assumed to be 1.0 psi/ft with a density of 19.23 lb/gal, an average weight of fluid-filled rock.
99宿舍四级成绩查询Normal formation pressure is equal to the hydrostatic pressure of the native formation fluids. In most cas, the fluids vary from fresh water with density of 8.33 lb/gal (0.433 psi/ft) to salt water with density of 9.0 lb/gal (0.465 psi/ft). However, some field reports indicate instances when the normal formation fluid density was greater than 9.0 lb/gal. Regardless of the fluid density, the normal pressure formation can be considered as an open hydraulic system where pressure can easily be communicated throughout.
Formation pressures resulting from under compaction often cab be approximated with some simple
calculations. If it is assumed that compaction does not occur below the barrier depth, the formation fluid below the barrier must support overburden, rock matrix and the formation fluids. The pressure can be calculated as:
P = 0.465 psi/ft (D B)+ 1.0 psi/ft (D i – D B)
D I = depth of interest below the barrier, ft
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D B = depth of barrier, ft
P = formation pressure at D i , psi
Figure 1.1 Abnormal pore pressures are generated in the under-compacted region becau the shale matrix can’t support the overburden stress
Example 1-1:
A well is drilled to 15000 ft. The entrance into the abnormal pressures at 10000 ft is caud by under compaction. Calculate the expected formation pressure at 15000 ft. Assume formation fluid and overburden stress gradients are 0.465 psi/ft and 1.0 psi/ft respectively?
Solution:
韩国的英文
The formation pressure at 15000 ft:
P = 0.465 psi/ft (D B)+ 1.0 psi/ft (D i – D B)
P = 0.465 psi/ft (10000)+ 1.0 psi/ft (15000– 10000)
P = 9650 psi
P = (9650 / 0,052 x 15000)
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P = 12.4 ppg
Log Analysis
Log analysis is a common procedure for pore pressure estimation in both off-t wells and the actual well drilling. New measurement while drilling (MWD) tools implement log analysis techniques in real time drilling mode.
The resistivity log was originally ud for pressure detection. The log respon is bad on the electrical resistivity of the total sample, which includes the rock matrix and the fluid-filled porosity. If a zone is penetrated that has abnormally high porosities ( at the same time high pressure) the resistivity of the rock will be reduced due to the greater conductivity of water than rock matrix. The expected respon can be en in Figure 1-2. This figure illustrates veral important points. Since the high formation pressures were originally developed in shale ctions and later equilized the sand zone pressures, only the clean shale ctions are ud as plotting points. This excludes sand resistivities, silty shale, lime and any other type of rock that may be encountered. As the shale resistivities are lected and plotted, a normal trend line should develop prior to entry into the pressured zone. An actual field ca can be en in Fig. 1-
2. The impermeable .shale ction was entered at about 9,500 ft. Although this ction contained normal pressure from 9,500-9,800 ft, as evidenced by the increasing resistivity of the normal trend, the reversal can be en from 9,800-10,900 ft. The mud weight was 9.0 lb/gal at 9.500 ft but was incread to 1
3.5 lb/gal at 10,900 ft.
Figure 1-2 Generalized shale resistivity plot
宽敞的意思Hottman and Johnson developed a technique bad on empirical relationships whereby an estimate of formation pressures could be made by noting the ratio between the obrved and normal rock resistivities. As they explained, the following steps arc necessary to estimate the formation pressure.
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1. The normal trend is established by plotting the logarithm of shale resistivity vs depth.
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2. The top of the pressured interval is found by noting the depth at which the plotted points diverge from the trend.
3. The pressure gradient at any depth is found as follows:
口蜜腹剑a) The ratio of the extrapolated normal shale resistivity to the obrved shale resistivity is determined.哒哒英语收费
b) The formation pressure corresponding to the calculated ratio is found from Fig. 1-3.
