Abnormal Pressure While Drilling Definition
By definition, abnormal pressure is any geo-pressure that is different from the established normal trend for the given area and depth. In this article we will introduce the major 6 causes of abnormal pressure while drilling. Also in the following articles we will discuss its prediction and more.
Pressure may be
- less than normal, called subnormal, or
- greater than normal, which has been termed geo-pressured, super-pressured, or simply abnormal pressured.
Subnormal pressures present few direct well control problems. However, subnormal pressures do cause many drilling and well planning problems. For clarity, the term abnormal pressure will identify the pressures greater than normal.
What Is The Formation Pressure
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Formation pressure is the presence of the fluids in the pore spaces of the rock matrix. These fluids are typically oil, 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 less 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 plastic rock.
What Is The Normal Formation Pressure
Normal formation pressure is equal to the hydrostatic pressure of the native formation fluids. In most cases, the fluids vary from fresh water with a density of 8.33 Ib/gal'(0.433 psi/ft) to salt water with a 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.
The Relation Between Entrapment & Abnormal Pressure
Abnormal formations do not have the freedom of pressure communications. If they did, the high pressures would rapidly dissipate and revert to normal pressures. Therefore, a pressure entrapment mechanism must be present before abnormal pressures can be generated and maintained. Fertl and Timko listed several of the more common entrapment seals throughout the world (Table 1).
Assuming that a pressure seal is present, the causes or origins of pressure depend on such items as lithology, mineralogy, tectonic action, and rate of sedimentation. Fertl lists many of the field-reported causes of high pressures (Table 2). Several of these causes will be discussed later.
1 Causes Of Abnormal Pressure While Drilling | Compaction of Sediments.
The normal sedimentation process involves the deposition of layers of various rock particles. As these layers continue to build depth and increase the overburden (total rock) pressure, the underlying sediments are forced downward under the weight of surface deposition. The overburden pressure in this case is defined as the total of the rock matrix pressure and the formation fluid pressure. Under normal drilling conditions, the formation fluid pressure is the main concern, due to its ability to cause fluid flow into the wellbore under certain geological conditions and the general inability of the rock matrix to move into the wellbore because of its semi-rigid structure.
The manner in which the rock matrix accepts the increasing overburden load explains the abnormal pressures generated in this environment. As both the surface deposition and the resultant total overburden increase, the underlying rock must accept the load.
Rock matrix strength can increase with an increase in grain-to-grain contact of the individual rock particles. This implies that the resultant porosity must decrease with depth under normal sedimentary conditions. This relationship can be seen in Figs. 3 and 4. If the normal porosity compaction process is prohibited by not letting the fluids in the pore spaces escape, the rock matrix cannot increase its grain-to-grain contact or its overburden support capabilities.
Since the total overburden load continues to increase with sedimentation and the rock matrix can no longer carry its burden, the fluids in the pores of the rock must begin to support part of the overburden, resulting in higher-than normal fluid pressures (Fig. 4). The depth at which the abnormally high porosities are encountered is usually assumed to be the initial depth at which abnormal pressures are encountered.
The Sealing Mechanism
A sealing mechanism must be present to trap the abnormal pressures in their environment. The most common sealing mechanism in continuous depositional basins is a low-permeability layer of rock, such as a clean shale section. The shale reduces normal fluid escape, causing undercompaction and abnormal fluid pressures.
Calculation Of Formations Pressure
After we understand how sediments compaction Causes Abnormal Pressure While Drilling, we must know that Formation pressures resulting from under-compaction often can 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 all overburden, rock matrix and formation fluids. The pressure can be calculated with Eq. 3.1:
EQ.1 : P = 0.465 psi/ft DB + 1.0 psi/ft (Dl – DB)
Dl = depth of interest below the barrier, ft
DB = depth of the barrier, ie., low-permeability section, ft
P = formation pressure at Dl psi
In Eq. 1, the overburden pressure gradient is assumed to be 1.0 psi/ft and the normal formation fluid pressure gradient is 0.465 psi/ft.
Eq. 1 can be used to approximate formation pressures. However, formations normally have some degree of compaction below the barrier. As a result, Eq. 1 can’t be expected to provide precise results in most cases. If necessary, a more complex series of calculations based on Eq. 1 can be used to increase the accuracy of the method. This complex procedure will not be presented.
2 Causes Of Abnormal Pressure While Drilling | Artesian Systems.
An artesian water system can create abnormal pressure if the proper structural conditions exist. As shown in Fig. 5, the continuous water-bearing sand will transmit hydrostatic pressures of the formation water to the bottom of the structure. The pressure at the top of the structure will be normal for the depth at which it is encountered. The pressure at the bottom of the structure will be equivalent to 13.5 Ib/gal mud weight. These pressures can not be detected with conventional techniques.
3 Causes Of Abnormal Pressure While Drilling | Uplift.
A normal pressure is defined in relation to the depth at which it is encountered. A pressure that is normal for a specific depth would be abnormally high for it shallower depth. Tectonic actions that uplift sections of formations can cause abnormal pressures in the uplifted section if specific formations within the uplifted section are sealed so the abnormal pressures cannot revert to normal during the course of geologic time. It is not uncommon to drill through a fault and enter a different pressure .environment. Caution must be exercised with respect to well planning because pressures across a fault line can be lower, as well as higher, than the pressures on the opposite side of the fault.
Fig. 6 illustrates the concept of abnormal pressures ,generated by uplifting. A 12.0-lb/gal mud will be required to drill the interval at 6,000 ft.
The sealed fault line. prevented a pressure regression to a normal environment.
4 Causes Of Abnormal Pressure While Drilling | Salt Beds.
Worldwide, salt beds are perhaps the dominant cause for formation pressures approaching the total overburden stress. Bedded salt is unique when compared to shale; it is impermeable, whereas shale is semipermeable. In addition, it behaves plastically, transmitting the complete overburden stress to the underlying formations. As shown in Fig. 7, the formations below the salt beds will require mud weights in excess of 19.0 Ib/gal.
5 Salt Diapirism (Salt Domes).
Salt diapirs, known as salt domes, result from a density inversion with a material of low shear strength. The plastic nature of the salt allows it to “flow” upward. This movement can over-compact formations in shallow sections, as shown in Fig. 8. This form of pressure generation creates unusually high formation pressures in shallow formations.
6 Density Differences.
Fluid density differences between zones with connecting permeability can cause abnormal pressures. The U-tube principle is the underlying basis for this. As shown in Fig. 9, a non-horizontal zone containing a low-density fluid requires greater-than-normal mud weights to drill the upper sections of the zone. Typical requirements for this occurrence are zones that have sealing mechanisms at the upper end and no permeability restrictions at the lower end.