Computation of cementation factor and saturation exponent for selected oil fields in southern Iraq

This study aims to calculate cementation factor and water saturation exponent for four different lithological formations from different oil fields in south of Iraq and compare the results with typical values of these reservoir parameters. The formations that being selected for this study are: Yamama Formation in West Quran (WQ-60 well), Zubair Formation in south Rumaila (Ru-64 well), Nahr Umr Formation in Luhais oil field (Lu-5 well), and Mishrif Formation in Tuba oil field (Tu-4 well). The cementation factor for clastic formations (Zubair and Nuhr Umr) was calculated via Wyllie (1949) and He (2005) empirical equations. It is found that the value of this parameter is estimated to be 1.1 and 1.90 m for the Zubair formation using Wyllie equation and is equal to 2.4 and 1.6 m for Nahr Umr Formation according to the He equation. For the carbonate formations (Yamama and Mishrif), the cementation factor was estimated using Borai (1987) and Focke and Munn (1987) equations. This parameter was equal to 1.3 for both formations according to Focke and Munn (1978) and estimated to be 1.95 and 1.98 for Yamama and Mishrif formations according to equation developed by the Borai. The calculated saturation exponent was 2.5, 2, 2.21 and 0.5 in Zubair, Nahr Umr, Mishrif, and Yamama formations, respectively. The new calculated values were then applied in the Archi equation to estimate water saturation. The obtained results were compared with that calculated in laboratory (from core). Results showed that Wyille equation is better than the He equation. The final result confirmed that Wyllie equation is more accurate than He equation for calculating cementation factor in sandstone, while Borai equation was more accurate than Focke and Munn equations for the cementation factor in limestone rocks. No.20 Journal of Petroleum Research & Studies (JPR&S) 114 قارعلا بونج يف ةراتخم ةيطفن لوقحل عبشتلا ساو ةتنمسلا لماعم باسح الله لام ةنمآ لظنح ، يباتعلا ةمعن نيسحلا دبع ، ديجم ديلو ةيداف ةرصبلا ةعماج ،مولعلا ةيلك ،ضرلاا ملع مسق ةصلاخلا ةفلتخم لوقح يف ةفلتخم ةيراخص تاذ نيواكت ةعبرلا عبشتلا ساو ةتنمسلا لماعم باسح ةساردلا هذه يف مت يتانوبراكلا ةماميلا نيوكت نيواكتلا هذه تنمضت .قارعلا بونج نم ةنرقلا برغ لقح يف (well WQ-60) نيوكتو يبونجلا ةليمرلا لقح يف يلمرلا ريبزلا (well Ru-64) يلمرلا رمع رهن نيوكت و سيحللا لقح يف (well Lu-5) ةبوطلا لقح يف يريجلا فرشم نيوكتو (well Tu-4) رمع رهنو ريبزلا امه ةيلمرلا نيواكنلل ةتنمسلا لماعم بسح . يتلداعم بسح (Wyllie,1949) و He (2005) ) ةتنمسلا لماعم ةميق نا دجوف (m ىلا ةيواسم 1.1 و 1.9 بسح ةلداعم Wyllie ىلا ةيواسمو ،يلاوتلا ىلع رمع رهنو ريبزلا ينيوكتل 2.4 و 1.6 ةلداعم بسح He ريبزلا ينيوكتل وكت امه ةيريجلا ةيراخصلا تاذ نيواكتلا ىلا ةبسنلاب اما .يلاوتلا ىلع رمع رهنو تبسحف فرشمو ةماميلا يني m امهل يتلداعم بسح (Borai,1987) و Focke and Munn , 1987) ةميق تدجوف ،( ) ةتنمسلا لماعم (m ىلا ةيواسم 1.3 ةلداعم بسح Focke and Munn , 1987 ىلا ةيواسم تدجوو ،نينيوكتلل 1.95 و 1.98 ةلداعم بسح Borai,1987 لع فرشملاو ةماميلا ينيوكتل عبشتلا سا بسحو .يلاوتلا ى (a) ىلا ةيواسم هتميق تدجوف 2.5 و 2 و 2.21 و 0.5 يجرا ةلداعم يف ةديدجلا ميقلا تقبط مث .يلاوتلا ىلع ةماميلاو فرشمو رمع رهنو ريبزلا نيواكت يف ةلداعم نا تدجوف ةبوسحملا جئاتنلا عم ةديدجلا جئاتنلا تمسرو يئاملا عيشتلا باسحل Wyllie م لضفا ةلداعم ن He ميق باسحل m ةلداعم تناك امنيب .ةيلمرلا روخصلا يف Borai ةلداعم نم لضفا Focke and Munn باسحل m .ةيريجلا روخصلا يف


Introduction
Reservoir water saturation (S w ) is an important factor for studying petroleum reservoir.
Calculation of S w is an essential step for computing oil saturation and in place oil accumulation. The simplest way for calculating S w is Archie formula, which is defined mathematically as: where, S w is water saturation, ɸ is porosity (fraction), R w is formation water resistivity, R t is true resistivity of the formation, a is tortuosity factor, m is cementation factor, and n is saturation exponent. m is a measure of the degree of cementation and consolidation of the rock. As the degree of consolidation increase, the value of the cementation factor increase too [1]. hydrocarbon and water saturation, which are indispensable parameters in the exploration of oil reservoir [2,3,4]. The poor estimates of the m can cause errors in the calculation of the S w when using Archie's equation and may lead to discrepancies between log interpretation and production test results [5]. In petrophysics routine evaluation, Archie's parameters are held constants with default saturation exponent equals to 2 [4]. In fact, n varies considerably from the default value of 2 in strongly water wet reservoir rocks to more than 20 in strongly oil wet reservoir rocks [6]. In addition, there are different factors affect on the m parameter such as shape of the grain, geometry of the pore system, grain size, tortuosity, grain size distribution, porosity, influence of pore geometry and wettability.
In this study, Archie's parameters were computed using different formula and compare with those calculated with default values.

Geological setting and stratigraphy
The Zubair Formation is assumed to represent a prograding delta originating from the Arabian shield [13,14]. The age of the formation as determined based on fossils is Hauterivian till early Aptian [9]. While palynomrphs evidence extended this formation up to earliest Albian age [15]. which consists of dark, slightly pyretic shales interbedded with pseudo -Oolitic detrital limestone [9], and this is overlain by Shuaiba Formation which consists of dolometic limestone.The Nahr -Umr Formation is composed of sandstone and shale. It is defined by Glynn Jones in 1948 in [9] from the Nahr -Umr structure in south Iraq. It is up to 360 m thick in the south parts of the Salman and Mesopotamian zones [7]. At its type section in the Nahr -Umr field, it comprises black shales interbedded with medium to fine grained sandstone containing lignite, amber and pyrite [12]. The Nahr -Umr Formation is interpreted to be an alluvial to lower coastal plain to deltaic deposit with shallow -marine and aeolian influences [16,17]. At Buzurgan, the formation includes glauconitic and bitumeinous sandstone and abmer [18].
Average TVD thickness of Nahr -Umr Formation is 260 m, Nahr Umr Formation is an oil reservoir in some fields. The upper contact of the Nahr Umr Formation with the overlying Mauddud Formation is conformable and gradational, and is placed at the base of the limestone of the Mauddud Limestone Formation and the top of the black shale of the Nahr Umr Formation [19]. The lower contact of the Nahr Umr Formation at the type section is with the Shuaiba Limestone Formation, where a disconformity was established on regional evidence [20]. The Mishrif Formation represents a heterogeneous formation originally described as organic detrital limestone, with beds of algal, rudist, and coral-reef limestone, capped by limonitic fresh water limestone [9].

Methodology:
In this study, a set of equations were used to calculate the values of cementation factor and saturation exponent according to the type of rock, they are listed below along with their authors.
x Formula for clean Sand stone (Wyllie, 1949) [21] and He (2005) [22] Adopted in the calculation of m on the value of index as following: x m= for 0.92 x m= for 0.92.
x Formula for carbonate m= 2.2-0.035/(Ø+0.042) (Borai, 1987) [23] x Formula to calculated the cementation factor, m, in limestone and for different ). This plot is usually yield a straight line with a slope equal to n. The calculated cementation factor and saturation exponent are used to calculate water saturation and the results were compared with that calculated using default values.

Results and discussion
When applying the equations for calculating a and m according to the rock type, the results confirmed that a is equal to 2.5 in the Zubair formation, while it is equal to 2. 2) The Borai equation for calculating the coefficient of m for carbonate formations is better than the Fock equation. 3