The Role of Chemistry of the Oil-Field Water in the Distribution of Reservoir Pressures: A Case Study of Mishrif Reservoir in the Southern Oil-Fields, Iraq

Mishrif Formation is the main reservoir in oil-fields (North Rumaila, South Rumaila, Majnoon, Zubair and West Qurna) which located at Basrah southern Iraq. The Inductively coupled plasma-Mass spectrometer (ICP-MS) was used for the water chemistry analysis and Scanning Electron Microprobe (SEM) for the purpose of mineralogy diagnosis. A weak acidic water of salinity six-time greater than seawater plays a role in generating the formation pressure and controlling the fluid flow. The potentiometric subsurface maps were modeled and the direction of super-pressure sites that are of a great importance in the oil exploration were marked to pay attention during future drilling.


No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
Mishrif reservoir [8&9]. Fluid movement often is studied based on the values of reservoir pressure by taking readings and ignoring the study of solution salinity. The hydrochemical studies seem to be few or not available in the Mishrif reservoir, particularly, those dealing with fluid-rock interactions. The dissolution and precipitation are the main chemical processes that determine the amount of salinity which is an effective factor in determining the flow path in the reservoir [9]. The aims of the present study are to define the type of the oil-field water, model the flow direction of the fluid in the reservoir using vector potentiometric maps and then pinpoint the high-pressure sites in the reservoir which serve drilling and prospecting processes.

Location and structure of the study area:
The studied oil-fields are located in Basrah, southern Iraq Figure (1). The dimensions (length and width) of these oil-fields are: West Qurna (35, 8km), Majnoon (48, 11 km), Zubair has three domes; Shuaiba (34, 17 km), Rafidiya (11, 8 km), and Safwan (4, 6 km) [10]. The West Qurna, North Rumaila and South Rumaila are considered as one structure according to the seismic data collected from the Oil South Company in 1987. The study

Materials and Methods:
The chemical composition and physical parameters of oil-field waters in the Mishrif Formation are studied in twenty-five oil wells; five water samples from each of the Rumaila North (R), West Qurna (WQ) and Majnoon (MJ), four samples from the Rumaila South (RU), and six samples from Zubair (ZB). The analysis was conducted by inductively coupled plasma-mass spectrometry (ICP-MS) technique in the laboratories of ALS, Spain.
The salinity potential maps were drawn using the surfer software to clarify the fluid path flow. The hydrochemical formula depends on cations and anions in epm in addition to pH and TDS [12] was expressed by the following equation: Equation calculated the concentrations more than 15%, and concentrations less than 15% were ignored.
Scanning Electron Microsprobe (SEM) was conducted to identify the mineralogy of the reservoir. E55 4. Results and discussion:

Reservoir hydrochemistry
The detailed chemistry results of the studied reservoir are presented in Table (1). The Na + , Ca 2+ , Cland SO 4 2ions compose of more than 90% of the total TDS; where ions descended as Na + > Ca 2+ > Mg 2+ > K + and Cl -> SO 4 2-> HCO 3 -, so the oil-field waters are dominated by Na and Cl. The Na content is seven times more than of seawater and ranges from 68779 ppm in WQ to 81895 ppm in MJ.
The Na availability in the oil-fields is attributed to the long-term water trapping period and sodium solubility, where [13] pointed out that the high sodium content in the brines related to its high mobility in the hydrosphere. Salt domes are well known in the study area and may contribute to add Na and Cl to the brines.
The Ca content shows twenty-eight times in comparison to the seawater, ranging from 9837 ppm in MJ to 12196 ppm in the WQ. The availability of Ca is a function of reservoir dissolution and calcium carbonate scale may be formed when is being oversaturated, where it is a most common scale found in plugged oil-field reservoirs [14].
The lack of Mg in brine is linked directly to dolomitization [15], a twice as much as seawater was recorded varying from 2031 ppm in MJ to 3091 ppm in WQ. The high Mg content indicates a low rate of dolomitization and dissolving of Mg-bearing minerals.
Potassium content increases in aqueous solutions under high temperature until the sylvite precipitates [16]. Potassium is found as five times as much in seawater, where the lowest content (897 ppm) in the ZB, and the higher (2366 ppm) in the RU. Shale is a responsible agent of K, particularly where containing illite. Chloride is a predominant ion in all oilfields, the lower average (131751 ppm) is in WQ, whilst the higher (153934 ppm) in MJ.

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
E56   Table   (2).  shoal and lagoonal complex is suggested to be a depositional model. To the west and south direction over Rumaila Formation, the shallow facies was developed [19]. After initial flooding of the platform, mixed shallow-water and planktonic facies developed on top of the platform [18].
The north part of the oil-field was developed with a shallow-water platform, followed by a coral-rudist build-up dominated platform. The south part of the oil field is characterized by the domination of mid-outer ramp restricted facies [19].

Potentiometric mapping model
A considerable variation in TDS (221660 WQ-253052 MJ ppm) has been recorded in the oil-field studied. The high salinity was due to a later diagenetic processes, where it increases at sites containing mudstone. The montmorillonite in the early diagenetic stage is one of the most important reasons for salinity increasing. Sodium and K are easy to be replaced by those higher ones such as Ca and Mg to form more stable montmorillonite compounds [20].
The Mishrif Formation consists of (top to base) a fine-grained limestone followed by dense fractured or stylolite turn into detrital porous partly very shelly to foraminiferal limestone with banks of rudist grades downwards into marly limestones [2]. The presence of marl and shale beds within the reservoir is one reason for the super salinity. Other reasons for salinity are salt domes nearby the shoreline.
The salt plug of the Jabal Sanam located 45 km southwest Basrah [21] may be considered as a potential source of salinity. Movement of the fluids in the subsurface environment depends on different variables, commonly density of the fluids, salinity, temperature and pressure [22].
The subsurface formation pressure produced by a variety of different mechanism may be physical, chemical or combination of both. The chemical factors which increase the pressure include the variety in the water density distribution owing to the salinity and temperature. The fluid path flow is from concentrated to less concentrated sites. The interstitial water must carry additional loads of ions to generate suitable subsurface pressure to move the fluids.
Increasing of the temperature in the reservoir will increase the pressure and contribute to the fluid movement [23]. The salinity dataset Table (

Conclusions
Through the studying of oil-filed water chemistry in five giant oil-fields in southern Iraq and its influence on the pressure distribution inside oil reservoir, the findings can be drawn as follows: 1. Sodium and chloride are the predominant ions in the all oil-fields, so the water type is Na-chloride, except for the WQ oil-field which characterized by an increase in the calcium content in addition to sodium and chloride, so it has Na-Ca-Chloride type. E62 2. The oil-field waters are characterized by high salinity, six times greater than seawater with relative variation; where the highest salinity was recorded in the MJ field and lowest salinity was recorded in WQ field.

No.22-(3) 2019 Journal of Petroleum Research & Studies (JPR&S)
3. The origin of high salinity is due to connate marine water and diagenesis processes and it is believed that salt domes contributed to increased salinity as there is evidence on the