Abnormal Pressure Distribution of Tertiary age formations in Middle & South Iraqi Oil Fields

Prediction of formation pore pressure gradient is a very important factor in designing drilling well program and it help to avoid many problems during drilling operations such as lost circulation, kick, blowout and other problems. In this study, abnormal formation pressure is classified into two types; abnormal high pressure (HP) and abnormal low pressure (LP), therefore any pressure that is either above or below the hydrostatic pressure is referred to as an abnormal formation pressure. This study concerns with abnormal formation pressure distribution and their effect on drilling operations in middle & south Iraqi oil fields. Abnormal formation pressure maps are drawn depending upon drilling evidence and problems. Three formations are considered as abnormal formations in the region of study, these formations geologically existed in Tertiary age and they from shallower to deeper are: Lower Fars, Dammam and Umm Er Radhuma, Formations. The maps of this study referred to either high formations pressure such as (Lower Fars and Umm Er Radhuma) or the low formations pressure such as (Dammam) in middle and south of Iraq. Finally these maps also suggested and showed the area, where no field is drill until now, which may behave as high, low and normal formation pressure for every formation understudy.


1-Introduction
Abnormal formation pressures are encountered throughout the world and it produced by many different causes that may be physical, chemical, or a combination of the two. Many reasons for the formation of subsurface abnormal fluid pressures have been postulated and discovered. Abnormally high pressured formations, recorded at depths of a few hundred feet to that greater than 20,000 ft. These formations have fluids were isolated environments or the fluid movement to flow out of the formation is restricted. The total overburden load is partially supported by the pore fluids. These abnormally formations can only exist if the formation is separated by impermeable barriers that contain the pressure in the reservoir [1].
Formations which exhibit a pressure deviation towards higher or lower formation pressure rather than the normal hydrostatic pressure commonly, because drilling problems.
Some of these problems are; lost circulation, lost hole, stuck drill pipe, flowing fluid (salt water or hydrocarbon) into well, blow out and casing setting problems ( at soft or fracture formations). Any one of these or together associated with abnormal pressure will increase drilling costs [2].
In Iraq, there is enough drilling experience to nominate the abnormal pressure formations. These formations make real challenges to well construction operations in most of Iraqi oil fields.
Pore pressures gradient can be divided into two forms normal pore pressure gradient and abnormal low and high pore pressure gradient. In fact, the formation pressures is low pressure (Subnormal) when its pressure is less than normal pressure gradient 0.465 psi/ft, while the formation pressure is high pressure (Overpressured) when its pressure is greater than normal pressure gradient 0.465 psi/ft. The reason which creates this abnormality in pore pressures may be very complex and vary from one region to another [3].
For that, it is very important to predict and detect abnormal pressure zones, where there are many risks such as loss of drilling fluids, kicks, blowout and other difficulties. In addition, it is also very important to predict the pressure at which the rocks will fracture. These fractures can result in losses of large volumes of drilling fluids. [3] Notable effects of high formation pressure that are costly include the following [4]: Blowouts uncontrolled production of formation fluids Caving high pore pressure in low permeability rocks causes them to stress relieve or Stuck pipe the drill pipe adheres to the side of the borehole due to the swelling (stress relief) of the borehole walls behind the bit Lost circulation the lost circulation may occur when raising the mud weight in order to control the formation pressure at the bit, the formation may rupture while hydrostatic pressure exceed formation pressure. The mud will then run out into a cavity of its own making.
When drilling in areas where high pressure zones are exist, it is necessary to be aware of both the pressure of the fluids in the pores and the pressure at which the formations willfracture. [4]. Subnormal (abnormal low) formations pressures, (less than 0.465 psi per ft of depth), are very common. The cause of abnormal formation low pressures is not well understood. If a formation containing either gas or oil is isolated and then subjected to uplift and erosion.
Subnormal formation pressured has much less attention than to high formation pressure. This is probably because there are fewer spectacular drilling problems associated with subnormal pressures and underpressures. However, problems exist that can be serious [5]. Drilling problems with Low (subnormally) pressured reservoirs [4,5] Much less attention has been paid to subnormally pressured reservoirs than to overpressured reservoirs. This is probably because there are fewer spectacular drilling problems associated with subnormal pressures and underpressures. However, problems exist that can be serious.

Causes of Overpressured Formation [2, 6]
If the reservoir pressure is much lower than the pressure of the drilling fluid, severe formation damage can occur. The drilling mud filtrate penetrates the reservoir, causing swelling and migration of clays, which may plug the pore throats. Even a little water in the hole can kill a low pressure producing gas well. The water is drawn into the pores by capillarity and ruins the relative permeability to gas. In the case of low pressure gas sandstone reservoirs, it is desirable to set casing at the top of the reservoir interval and drill with gas, salt water, or oil-based mud to minimize formation damage.
Also, if the gas reservoir has a low pressure, there may be no indication of gas on the mudlog. The logs of many abandoned dry holes should be reexamined to look for bypassed gas zones.

Area of Study
The basic map for this study was designed at geology section in Exploration Oil Company in the Ministry of Oil. Finally, the locations for 35 fields (one well for each field) are established into basic map as shown in figure (1) and listed in table (1).

Fig. (1) Map shows all wells under study
The data used in this study to draw and establish Formation pressure distribution maps were obtained from final well reports, final drilling reports and final geological reports.
Abnormal low pressure and abnormal high pressure formations were indicated from mud losses and flowing (water and gas) events. Mud density of wells under study and Stratigraphy columns of 35 fields in middle-southern region of Iraq were studied as well.
This study includes both shallow and deep wells, and includes many exploration and production wells in order to take the larger area with different locations that permit to compare and connect between the same formations which have the same behavior of abnormality pressure (high or low). Table (1) gives the range of depth which is considered in the present study for each well.

1-Results and discussion
In this research, the work focused on finding a way to draw the distribution of abnormal

ENVI ( "Environment for Visualizing Images")
It is a software application used to process and analyze geospatial imagery. It is commonly used by remote sensing professionals and image analysts. ENVI combines advanced image processing and proven geospatial analysis technology to help in extract meaningful information and make better decisions [10].
This program has been used to draw a map that shows the basic study area (southern and central Iraq), and also to install locations of wells according to UTM coordinate (Northing & Easting) system which have been identified from final drilling and geological well reports.
The maps are divided in two kinds, first; on the basis of high pressures (upper than normal trend, may be called Overpressure or Geopressure). This case causes the entry and flow of fluids into the well, which may cause a change in the density of the drilling fluid, which may lead to a kicked well problem or blow out. In some cases when fluid which entered the well is in high density which will increase hydrostatic pressure against the drilled formation then causing drilling fluid loss problems. Second; on the basis of Low pressures (less than normal trend, may be called Subpressure). This case causes of the entry and flow of the drilling mud into the formation, then lost circulation problems. In some cases it results in a reverse process like a kickoff problem. In this case the drilling fluid which losses into formation leads to reduce the hydrostatic column of drilling mud then reduce the hydrostatic pressure against formation, thus formation pressure overcomes on hydrostatic pressure and formation fluid enters the well.
In fact, oilfield drilling operations in central and southern Iraq accompanied by several problems so that these problems vary from one to another field depending on the sequence of the formations and their pore pressure.
Many exploratory wells were selected in this study because they can: 1-Give aclear idea of real problems that have been occurred when drilling these wells.
2-These wells drilled as deeper wells, approximately, the depth of these wells range between (1800-5400) meters.
Other production and appraisal wells were selected with consideration of their depth and the importance of the field in middle and south of Iraq.
The review of these formations is the aim of the current study; finally, these formations are drawn and illustrated the distribution of abnormal pressure in maps which designed for the first time.

2-Formations Under Study
Three formations selected in this research for studying and for drawing pressure distribution maps, which existed specially in geological sequence of Tertiary age, depending upon drilling evidence and problems. These formations from shallower to deeper are: Lower Fars, Dammam and Umm Er Radhuma as follows:  In the wide area the deposition of salt can be occurred. In fact, the salt sections usually considered as impermeable sections for fluid movement, therefore the underlying formations are become in high pressure. The zones which are found directly below a salt layer, usually it will be in high pressure [6].

4-2-Dammam Formation
This formation is existed within the geological sequence of the Tertiary/Eocene age  A low pressured zone may result when hydrocarbons or water are produced from a formation in which no aquifer is founded. This case is take place especially during drilling a development wells through a reservoir which has already been producing for some time [6].

4-3 Umm Er Rudhuma Formation
This formation is existed within the geological sequence of the Tertiary/Paleocene age inside Iraq [13]. It is exist in south of Iraq, in Basrah province wells (KI-1, Lu-9, RT-3, Ru-2, R- Umm Er Radhuma formation is acting as high pressure and the probability of sulfur water flowing is very high, these wells defined with dark red color while the wells that expect to behave as abnormality high pressure defined with light red color such as (Sa-1, Rt-3, R-172 and Mj-3).Umm Er Radhuma formation is shown the abnormality high pressure behavior only at (KI-1, Lu-9, WQ-15, Ru-2, Tu-3, NR-6, Dn-1, Sw-1, AK-1 and Su-10) because this formation contain sulfurs water at these fields. As shown in Figure (4). The migration of fluids from high pressure zones to low pressure zones at shallower depth may lead to this phenomenon. This may be caused by a poor job of casing cementing or occurred such as flowing fluids, kick and blow out especially at no lithology change. If the sands are charge by gas from lower formations, they will lead to high pressures zones [6].

Conclusions
These maps created to determine the expect area of high pressure and low pressure of Tertiary geological sequence formations. The following conclusion is obtained by this study: 1.The Lower Fars formation is shown the abnormality high pressure especially in Missan province fields because this formation have salt and shale sections in its composition. The highest formation value of formation pressure is 2.28 gm/cc and located at Bu and AG.
2.Dammam formation showed abnormal low pressure at all fields understudy.
3.In some fields, Umm Er Radhuma formations showed the problems of losses while they mainly have abnormal high pressure (usually contain sulfur water), this problems occurred when used high mud weight to overcomes these abnormality.

Recommendations
1. When additional data to be provided such as resistivity, conductivity data and daily drilling reports, other methods can be used to predict both pore and fracture pressure gradient.
2. It is recommended to insert other wells in western of Iraq to create a clearest idea about pressure behavior in this region.