Keywords

1 Introduction

1.1 Characteristics of Carbonate Reservoirs in the Middle East

The Middle East is extremely rich in oil and gas resources, accounting for up to 2/3 of the global crude oil reserves, while 80% of the oil and gas reserves in the Middle East are distributed in large, massive bioclastic limestone reservoirs [1]. The efficient development of bioclastic limestone reserves on a broad scale is dependent on the global oil and gas energy pattern, as well as a consistent and reliable supply of oil and gas.

The Alpine tectonic movement has largely influenced the Persian Gulf Basin in the Middle East, resulting in platform-type deposits [2, 3]. From west to east, the platform region in Iraq can be separated into two broad structural areas: the stable shelf area and the unstable shelf area. The latter is primarily distinguished by the formation of thick sedimentary capping. A succession of primary faults in the northwest-south or northeast-south direction determines the tectonic layout of Iraq's basement. Fault blocks and sub-fault blocks can be found in the basement. It is further classified into main belts and sub-belts based on the degree of structural distortion. In the unstable shelf area to the east, there are three primary structural belts: Mesopotamian belt, piedmont belt, and fold deformation belt [4]. The Zubeirian zone, the Euphratesian zone, and the Tigrisian zone are subdivided within the Mesopotamian zone. In terms of structural zoning, the Halfaya Oilfield is located in the Mesopotamia Basin's foredeep zone.

The Persian Gulf Basin has always been the basement of pre-Cambrian crystalline metamorphic rocks and metamorphic complexes located on the northern margin of the old Gondwana continent, despite multiple merges and separations between the global plates from the Cambrian to the Mesozoic periods. The Paleo-Tethys Sea is located to the northeast. The basin is mostly known for platform-type deposits, with caprock deposits dating back to the Cambrian period. There are rather thick deposits of the Hormuz Formation of the Ecambrian in the southern and western regions of the eastern unstable shelf area [5]. The western section of the Persian Gulf Basin has been primarily a long-term positive topography that has exposed the surface, which is the eastern part, during the Paleozoic, notably the Early Paleozoic. In the northeast, platform terrestrial and neritic deposits give provenance. At the time, the area was a shallow and wide surface sea, with many phases of transgression and regression, uplift and denudation, and multiple periods of sedimentary discontinuities, resulting in deposits primarily composed of clastic facies sediments. During the Paleozoic, outer platforms and platform areas were not influenced by orogeny, but only by land-making activities, which resulted in local outcrops and denudation, as well as fault depression and fault-block activity [6]. The Indian plate (central Iran, northwestern Iran, etc.) eventually shifted from Arabia along the Zagros line during the Triassic, due to the closure of the Paleo-Tethys Sea and subsequent crustal thinning events, as well as the extension of the Neo-Tethys Sea. The northeast corner is split.

Halfaya oilfield is 35 km southeast of Amara city in Iraq's Missan province. In 1976, Halfaya field was discovered. 2D seismic data from 1976 and 1980 was used to define the structure. The deepest well, HF-2, went down to the Lower Cretaceous Sulaiy formation at a depth of 4,788 m. Multiple Tertiary and Cretaceous reservoirs have been discovered with significant oil accumulations. Jeribe-Upper Kirkuk, Hartha, Sadi B, Tanuma, Khasib, Mishrif, Nahr Umr B, and Yamama are among the 10 reservoirs discovered so far in the Halfaya oil field's nine formations. Carbonate reservoirs with limited permeability predominate in these areas. However, high-permeability sandstone reservoirs have also been discovered [7]. In vertical and lateral orientations, both types of reservoirs exhibit significant heterogeneity. And Mishrif is the primary research reservoir in this study because it contributes the most to overall production and plays a vital role in waterflooding evaluation and production analysis for water breakthrough type classification in carbonate reservoirs (Fig. 1).

Fig. 1.
figure 1

Location of Halfaya Oilfield, Missan, Iraq

Full size image

1.2 Background

Geology Description.

The Mishrif (MB1-MC1) reservoir is classified as a huge reservoir with an edge-bottom aquifer. The regional first-order sequence boundary, MA1, of Mishrif, is made up of brecciated lime mudstone and packstone. The oil-bearing MA2 is made up of dolomitized packstone with biomorphic/dissolution pores that was deposited and diagenetically developed in a shoal environment. Except for the area around well HF-1, where rocks become fairly tight, the reservoir extends with a thickness of 6–11 m throughout the field. The reservoir porosity is between 13 and 16%, while the permeability is between 0.8 and 2.7 md.

Back shoal skeleton packstone interbedded with lagoon wackestone make up the majority of MB1. Pore types include mouldic pores, micropores, and visceral foramen, among others. With a thickness of 76–88 m, the reservoir is continuous. The reservoir has a porosity of 13–18% and a permeability of 1.9–3.1 md. In most regions, the highest interval of MB1 has tight limestone with a thickness of 4–27 m, which can act as a barrier between MA2 and MB1.

MB2 is primarily rudist grainstone and packstone that was deposited and diagenetically developed in a shoal setting. Moldic pore/dissolution pore is the most common pore type. Some regions have developed vugs. With a thickness of 40–50 m, the reservoir is continuous. The reservoir has a porosity of 19–25% and a permeability of 4.4–34 md. The lower sublayer, MB2–3, has a substantially higher porosity than the above two sublayers, MB2-1 and MB2-2, as seen in the cores of HF005-M316.

Foram packstone, rudist packstone, rudist grainstone, and dolomitic packstone deposited in a shoal environment make up MC1-1 and MC1-2, which are also oil-bearing zones. The majority of pores are mouldic/dissolution pores. In both MC1-1 and MC1-2, the reservoir thickness is 40–45 m. For MC1-1 and MC1-2, the average reservoir porosity is roughly 23% and 17.33%, respectively.

MC1-3 is primarily made up of skeleton wackestone and argillaceous skeletal wackestone, both of which were deposited and diagenetically created in a lagoon environment. Except in the region of well HF-2, these non-reservoir MC1-3 rocks, which have a thickness of 2–20 m in most areas, can act as a vertical fluid barrier to the water-bearing MC1-4 zone beneath. In addition, there are 20–30 m thick stable barriers between MC1 and MC2. Aquifer strength in Mishrif could be limited because to the presence of low-permeability interbeds.

The Mishrif reservoirs are oil wettability. For MB1, there are no or minor sensitivities to salinity, water, velocity, or alkalinity. Sensitivities to velocity (impaired permeability) and acid (improved permeability) are projected to be moderate to strong for MB2, whereas sensitivity to alkaline is expected to below (Fig. 2).

Fig. 2.
figure 2

Reservoir profile of Mishrif (Including MA-MC1)

Full size image

Production History.

Since 2012, the Mishrif reservoir has been formally put into production. It has generally gone through three stages of establishing production capacity. With a production size of 70,000 barrels per day, the first stage of production was mostly concentrated in the crest area of the reservoir's middle part. The second stage was in the southeast of the reservoir, where production scaled up to over 130,000 barrels per day, and the third stage was in the northwestern and peripheral regions of the reservoir, where production scaled up to around 200,000 barrels per day.

With the gradual development and production of oil reservoirs, the pressure of the reservoir has gradually decreased. Since 2015, the reservoir started a water injection pilot test and reached the scale of water injection in 2018. After three years of scale water injection, the trend of reservoir pressure decline has slowed down. However, more and more problems have erupted in reservoirs. The main problem is the increase in water cut of the reservoir, which has led to more and more water breakthrough wells and a rapid decline in single-well production. Although the overall water cut of the reservoir is still in the low water cut range, the water cut of several individual wells has exceeded 90%. Through reservoir water injection dynamic analysis, it has been confirmed that dozens of oil wells are in the high water cut stage. The causes of water breakthrough in oil wells are analyzed thoroughly to find out the causes of water breakthrough of different types of water breakthroughs, and treatment measures for different types of water breakthrough wells are proposed to achieve the purpose of controlling water cut and increasing oil production (Fig. 3).

Fig. 3.
figure 3

Production history of Mishrif reservoir

Full size image

2 Methodology

2.1 Static and Dynamic Integration Research

Static Research.

The huge and massive bioclastic limestone reservoir has high single-well production and good development benefits, but at the same time, it is accompanied by problems such as uneven production and differential water influx in multiple types [8,9,10,11,12]. The contradiction between internal water injection and water channeling is very prominent, which seriously affects the overall production of the reservoir. The production and production capacity continues to be exerted, and the root cause is analyzed, which is caused by the extremely strong heterogeneity of the oil reservoir.

Geological Study.

The MB2 large bioclastic limestone reservoir has a porosity of 20%–30% and a permeability of 1 mD–1000 mD, with high intralayer variability and low reservoir production. To improve reservoir classification and evaluation, clarify reserve production, clarify development potential, and integrate lithology, lithofacies, oil-bearing properties, physical properties, logging response, PLT, and development performance, among other factors, to develop rock type effective reservoir static and dynamic classification standards It is separated into microfacies and lithofacies based on bioclastic content, bioclastic size, mortar fills, and secondary minerals to define the types, distribution rules, and production characteristics of distinct lithofacies reservoirs.

Seismic Prediction.

The Mishrif reservoir's MB1-2 is created mostly in a constrained platform sedimentary environment with high reserves, however the single layer thickness is thin, physical qualities change frequently, and thin interbeds predominate. The seismic forward simulation records of different thin interbedded combinations have been established, revealing the seismic response characteristics of different scales and different combinations of reservoirs, based on the identification of thinly interbedded reservoirs and interbedded single wells, using seismic inversion technology. Combining the lateral superimposition and pre-product development characteristics of the platform margin beach reservoirs with the characteristics of the distribution of sedimentary facies belts controlled by the beach body position and tidal water channels, comprehensive reflection coefficient inversion, and other techniques to target the massive bioclastic limestone reservoir in MB2 of the Mishrif reservoir, comprehensive reflection coefficient inversion, and other techniques to target the massive bioclastic limestone reservoir in MB2 of the Mishrif reservoir.

Logging Interpretation.

Logging comprehensive interpretation is the most effective and direct method to identify barriers and high-permeability bands. Barriers and high-permeability bands are decisive factors for the water injection development and fine injection-production control of the stratified system. The use of production test data such as logging, MDT, and PLT to realize the classification of the entire reservoir and the evaluation of the production level reveals that the production contribution mainly comes from the high-quality reservoirs of Type I and II, while the Type III and IV reservoirs are basically unused. According to this, formulate effective development strategies for Class III and IV reserves and release the development potential of weakly produced reserves.

Sedimentary Facies.

The MC1-2, MC1-1, and MB2-3 layers have superimposed four stages of shoal facies in the form of platform margin accretion in the process of the sea level falling again. Detritus beach of thick clam, bioclastic beach, and second stage detritus beach of thick clam. In the second stage, the sedimentation rate of the clastic clam shoal was too fast, resulting in multiple exposures, and two layers of carbonaceous marl deposited in the closed water body formed during the exposure period. After the sea level rose in a small area, the MB2-1 layer of relatively stable crumb beach was deposited. The MB1 layer includes the MB1-1 layer and the MB1-2 layer. The MB1-1 layer is relatively fine in lithology, with marl and micrite limestone developed, and its top boundary is in contact with the MA layer with gray-green breccia. The typical sedimentary structure of MB1-1 includes dissolution and collapse above the sequence interface, deformed structure, and breccia structure. The MB1-1 layer also develops typical lumpy micrite limestone affected by tidal action. The gravel debris may be microcrystalline calcite, and the inter-grain is gray-green stucco. Therefore, the MB1-1 layer is a low-energy environment where shallow water is affected by tides. The thickness of the MB1-2 layer is close to 100 m, with bioclastic mudstone, bioclastic marl, and marl, which are frequently interlayered. Among them, bioclastic marl and mud limestone occupy this layer. The total thickness is about 70%. Among them, the bioclastic marl contains more environment-limited organisms, mainly benthic foraminifera, but a small number of thick clams, corals, red algae, and other organisms can be seen in local layers, as well as clastic limestone interlayers. These normal marine organisms imported from outside may be brought in by the shore storm surge, especially the clastics in the clastic limestone interlayer have the characteristics of directional arrangement. The marl in the MB1-2 layer is white, in which a large number of biological disturbance structures are developed, which are porphyritic. Biological disturbances usually occur in shallow water environments above the average low tide line. The MB1-2 layer develops not only bioclastic marl and argillaceous limestone with limited environmental beach facies, but also develops shallow water marl deposits and frequently interbeds, and the proportion of beach facies limestone is larger, indicating that this layer At the time of deposition, the study area was in the interactive development area of shoal facies and non-shoal facies, and the sedimentary environment was an alternate deposition environment of shallow water-limited shoal facies and non-shoal facies.

Dynamic Research.

The extremely thick bioclastic limestone reservoir is extremely heterogeneous, and the distribution of interlayers and high-permeability layers is complex, which leads to serious interlayer and intralayer interference in the reservoir, poor water flooding effect, and uneven production of reserves of different qualities. Reservoir dynamic analysis is the beginning of solving all problems, and this is also true for water breakthroughs in oil wells.

Performance Analysis.

In the specific analysis of the cause of water breakthrough in a certain well, firstly, it is based on the previous comprehensive geological-seismic-logging research results, combined with the actual production history curve of a single well, the drilling and completion history, and the operation history to eliminate cementing. After the quality of ground engineering reasons, a preliminary judgment of the possible cause of the water breakthrough. And preliminary judgment, the direction of water breakthrough, and the interval of water breakthrough.

Surveillance Analysis.

Through various types of logging before and after the water in the water well, such as PLT, ion, and salinity analysis, PBU or PTA test, etc., it is preliminarily judged whether the source of the water produced is from river water, formation water, or injected water. If the current monitoring data are not sufficient to support the analysis of the cause of water breakthrough, the missing important test data can be appropriately put forward, and the corresponding test can be completed as soon as possible to efficiently improve the efficiency of the analysis, and further rely on the actual water interval and direction of water breakthrough.

Simulation Model.

The numerical simulation model is a comprehensive integration that incorporates various static and dynamic information. A well history matched model is a powerful weapon for analyzing water breakthrough in oil wells. According to calculation and analysis of material balance and streamline models, it can assist in judging the source of water production and determining the water influx direction. At the same time, the numerical model also has the ability to predict the water cut trend of water breakthrough oil wells or the oil wells that may encounter water breakthrough in the future.

3 Case Study

The process of determining the type of water breakthrough: First, based on the comprehensive understanding of the geological reservoir, judge the possible type of water breakthrough, and demonstrate the source of the water breakthrough according to the analysis of ion salinity, whether it is river water, formation water, or injected water, through PLT/ILT and logging interpretation determine the chief injection-production intervals and predict the connectivity between wells through seismic and sedimentary facies. Based on the assessment of the produced water source and the direction of the water breakthrough, the type of water breakthrough is divided.

According to the direction of water breakthrough and the water breakthrough source, through water breakthrough wells analysis in the whole reservoir, the water breakthrough types of the Mishrif reservoir water breakthrough wells are divided into four types: bottom water coning type, edge water intrusion type, injected water channel type, and mixed water type.

Take M324 as an example to determine the water source and direction of water breakthrough. Firstly, the rapid increase in water cut is confirmed by its production curve. According to the injection-production well pattern, M324 is located in the crestal high part of the reservoir, and there are two water injectors around it. The vertical connectivity between wells is judged by seismic profile, the lateral connectivity among wells is assessed according to the sedimentary facies, and the spatial producing and injecting relationship is synthesized to preliminarily judge the possible water breakthrough intervals. According to its ion and salinity curves showing a swift downward trend, the preliminary judgment is injection water because the salinity of formation water is in the range of 100,000 to 200,000 ppm, and the source of injection water for oil reservoirs is river water, produced water, formation water, or mix, which is less than formation water.

During the rapid increase in water content of M324, M041D1 always maintains a relatively high injection volume, while M325 has a gradual decrease in water injection volume (see Figs. 4 and 5).

Fig. 4.
figure 4

Production and injection curves of M324 and M325

Full size image
Fig. 5.
figure 5

Production and injection curves of M324 and M041D1

Full size image

It can be seen from the sedimentary facies distribution that the connectivity between M324 and the two injectors is relatively good (see Fig. 6), and it can be seen from the seismic profile (see Fig. 7), for M041D1 and M324, MB1-2B, MB1-2C, MB2-1, and MC1 reveal better connectivity, while for M325 and M324, MB1-2C and MC1 reveal better connectivity.

Fig. 6.
figure 6

Sedimentary facies distribution of M324 M325 and M041D1

Full size image
Fig. 7.
figure 7

Seismic profile of M324 M325 and M041D1

Full size image

From the curves of chloride ion and salinity (see Fig. 8 and Fig. 9), it can be seen that M041D1 always injects river water with low salinity, which is consistent with the rapid decline of M324, while the curve of M325 shows that its salinity maintains a certain range, and Higher than the minimum value of M324. It can be judged that the water source is the injected water from M041D1 with a high probability.

At the same time, from the material balance of the numerical simulation (see Table 1), about 18% of the water produced by M324 comes from M041D1, while only 2% of the produced water comes from M325. This result further confirms that the injected water of M324 comes from M041D1.

Fig. 8.
figure 8

Salinity and chloride monitoring of M041D1 and M324

Full size image
Fig. 9.
figure 9

Salinity and chloride monitoring of M325 and M324

Full size image
Table 1. Material balance from numerical simulation for M324.
Full size table

4 Conclusion

In this study, a complete discriminating approach incorporating seismic-geology-logging-monitoring-dynamics-simulation is developed to determine the type of water breakthrough encountered by high water-cut wells. In Mishrif reservoir, three forms of water breakthroughs have been identified: severe bottom water coning at the bottom, swift injected water breakthrough, and layered edge water invasion.

Based on a thorough examination of all water breakthrough wells in the reservoir, it is considered that the reservoir's water breakthrough wells are mostly of the bottom water coning type, with injected water breakthrough and edge water invasion as secondary types. MB2 strata are the most common in bottom water. The injected water breakthrough is concentrated mainly in the high crestal area of MB1-2 and is primarily influenced by the reservoir's dominant phase's spatial distribution. Edge water invasion mainly occurs in flank areas of the southeast of MB1-2.

The water cut is generally decreased by plugging the major watering intervals or restricting the liquid production for wells that have been confirmed as bottom water coning. The injection and production adjustment of injected water breakthrough wells may be optimized, and the edge water invasion type can be enhanced by targeted plugging in the lower part of the watering zone and perforating the higher part to regulate the water cut and raise oil production.