Carbonate Karstified Oil Fields Geological Prediction and Dynamic Simulation Through Equivalent Relative Permeability Curves

Karst systems heterogeneity may become a nightmare for reservoir modelers in predicting presence, spatial distribution, impact on formation petrophysical characteristics, and particularly in dynamic behaviour prediction. Moreover, the very high resolution required to describe in detail the phenomena does not reconcile with the geo-cellular model resolution typically used for reservoir simulation. The scope of the work is to present an effective approach to predict karst presence and model it dynamically. Karst presence recognition started from the analysis of anomalous well behaviour and potential sources of precursors (logs, drilling evidence, etc.) to derive concepts for karst reservoir model. This first demanding step implies then characterizing each cell classified as karstified in terms of petrophysical parameters. In a two-phase flow, karst brings to fast travelling of water which leaves the matrix almost unswept. This feature was characterized through dedicated fine simulations, leading to an upscaling of relative permeability curves for a single porosity formulation. The workflow was applied to a carbonate giant field with a long production history under waterflood development. Firstly, a machine learning algorithm was trained to recognize karst features based on log response, seismic attributes, and well dynamic evidence, then a karst probability volume was generated and utilized to predict the karst presence in the field. Karst characterization just in terms of porosity and permeability is sufficient to model the reservoir when still in single phase, however it fails to reproduce observed water production. Karst provides a high permeability path for water transport: classical history match approaches, such as the introduction of permeability multipliers, proved to be ineffective in reproducing the water breakthrough timing and growth rate. In fact, the reservoir consists of two systems, matrix, and karst: however, the karst is less known and laboratory analysis shows relative permeability only for the matrix medium. The introduction of equivalent or pseudo-relative permeability curves, accounting for both the media, was crucial for correct modelling of the reservoir underlying dynamics, allowing a proper reproduction of water breakthrough timing and water cut (WCT) trends. The implementation of a dedicated pseudo relative permeability curve dedicated to karstified cells allowed to replicate early water arrival, thus bringing to a correct prediction of oil and water rates, also highlighting the presence of bypassed oil associated with water circuiting, particularly in presence of highly karstified cells.

Research progress and prospect of Ordovician carbonate rocks in Tahe oilfield: karst feature

Inspired by structural geology, karstology, geomorphology, as well as petroleum geology, coupling excellent documented research works and the field investigation, the status and prospect for the studies on karst feature of Tahe oilfield are discussed comprehensively. Results indicate: (a) the majority of research works have appeared since 2000 and can be classified as three categories and 12 subclasses, in which caves, fracture cave systems and fault-karst reservoirs are considered as the main research topics, (b) existed studies can be divided into four stages, and fault-karst reservoir analysis is a hot spot recently, focusing on the scale of the formed reservoir. It is the common sense that faults control both fractures and caves. Originally, there exist close relationships among karst phenomena whether they're on the ground or underground. Revealing these relationships mentioned above is the important direction for the studies of Tahe oilfield in the future. As for the areas where faults are widely distributed in karst, there exist hierarchical properties in karstology: (1) fault controls topography and landform, (2) fault, topography and landform control the water system and (3) fault, topography, landform and water system control karst. These hierarchical properties are the basic connotation of the laws for karst evolution, and the vital goals of the karst study of Tahe oilfield as well. Applications of methods and techniques and field-karst investigation are the solid guarantees to achieve these goals.

The Omniscient Spring – the Whispering Water: Man and potajnica

Potajnice (intermittent/rhythmic springs) are a rare hydrographic phenomenon exclusively present in karst areas. They appear as a result of the rising and emptying of the siphon in the underground karst reservoir, while the phases when the water flows over and drains down most frequently last for several minutes or several hours. They are often accompanied by a distinct noise (rumbling, sizzling, bursting) which adds to the mysteriousness of this phenomenon. Up to this point in time, around one hundred of these flows have been detected worldwide and, when it comes to Europe, they are most frequent in the Balkan Peninsula, primarily in the range of the Dinarides. The subject of this paper is the human perception of this unusual hydrological phenomenon and reaction to it, which has brought about the birth of many interesting ceremonies and beliefs. In areas with predominantly Orthodox and Islamic traditions, the water from these springs is often regarded as healing and miraculous, which is not the case in the northwestern areas closer to Central Europe. Nevertheless, the entire area is eager to provide a logical explanation for this phenomenon. In the more distant past, some of these places had been subject to Christianisation through construction of churches in their vicinity and provision of a Christian interpretation of local beliefs and tales. Man interpreted the functioning of these flows in two ways – either as the existence of a supernatural being in the underground canal, which occasionally presents a barrier to the water flow, or as activities of a higher power which infallibly judges the moral righteousness of human beings and thereupon decides to either let the water flow or stops its flow. The ancient human tendency to tame the supernatural has enticed man to look for a way to predict the flow intervals which, under certain circumstances, could ensure or confirm social power for an individual or group. At some point, the monks took over the perennial monitoring of the flow rhythm of one such source in order to harmonise their prayers for the appearance of the “healingˮ water. The relation between man and the potajnica has always been ambivalent; therefore, one river, which for centuries was called Sveto vrelo (the Holy Spring), also has an alternative name – Đavolje vrelo (the Devil's Spring). Also, men tend to cross significant distances to reach water from one spring, but avoid drinking water from others precisely due to the identical disappearance of the flow. Nevertheless, these springs represent unrivalled points where the richness of the spirit and tradition of the population visiting them is embedded, and, more generally, the relationship between man, as the human being, and this enigmatic force of nature.

New Method for Characterizing Internal Structure of Fault-Karst Reservoirs and Analysis on Acidizing Fracturing Effect: A Case Study in HLHT Oilfield, Tarim Basin, NW China

Superimposed tectonic movement and karst erosion resulted in a combination of fractures and irregular caves in deep/ultradeep carbonate rocks, typically along major fault swarms. Outlining these fault-karst reservoirs mainly depends on recognizing the strong reflection in seismic profiles; however, characterizing their internal structures is still difficult, which are represented as weak amplitude in seismic profiles. This study intended to propose a method to dissect the internal structure of fault-karst reservoirs, which contains four steps: (1) elimination of the signal interference by the covering bed with strong energy and recognition of internal reservoirs with low energy based on seismic data conversion, frequency division, and inversion; (2) gradient structure tensor analysis based on an anisotropic Gaussian filter for fault-karst reservoir outlining; (3) internal faults and cave recognition on the basis of wave-based inversion; and (4) reevaluation of the number and scale of these faults and caves based on seismic recognition and well test results and verification of their volumes and hydrocarbon reserves. The method was used in the evaluation of the fault-karst reservoir in the Halahatang (HLHT) oilfield, which is located in the north of Tarim Basin. The results show that the fault-karst reservoirs along major faults and their internal structures are effectively recognized, and the error of the predicted depth of the reservoirs decreases from more than 20 m before to less than 4 m now; the drilling success ratio increases from 70% to 90%. In addition, the method supports the recognition of untapped fault-karst reservoirs around shut-in wells, which provides guidance for sidetracking plans. Further, by comparing the geophysical volume of fault-karst reservoirs and the reserve predicted by production performance, the untapped reserve in a certain reservoir can be evaluated; on this basis, producing wells received high yields by targeted acid fracturing. In summary, the method effectively improves the prediction accuracy and the recovery efficiency of fault-karst reservoirs.

Diffraction imaging for fault-karst structure by least-squares reverse time migration

The fault-karst reservoir, which evolved from the deformation and karstification of carbonate rock, is one of the most important reservoir types in western China. Along the deep-seated fault zones, there are a lot widely spread and densely distributed fractures and vugs. The energy of the diffractions generated by heterogeneous structures, such as faults, fractures and vugs, are much weaker than that of the reflections produced by continuous formation interface. When using conventional full wavefield imaging method, the imaging results of continuous layers usually cover small-scale heterogeneities. Given that, we use plane-wave destruction (PWD) filter to separate the diffractions from the full data and image the separated diffractions using least-squares reverse time migration (LSRTM) method. We use several numerical examples to demonstrate that the newly developed diffractions LSRTM (D-LSRTM) can improve the definition of the heterogeneous structures, characterize the configuration and internal structure of the fault-karst structure well and enhance the interpretation accuracy for fault-karst reservoir.

Taking into Account both Explicit Conduits and the Unsaturated Zone in Karst Reservoir Hybrid Models: Impact on the Outlet Hydrograph

The main outlets of karst systems are springs, the hydrographs of which are largely affected by flow processes in the unsaturated zone. These processes differ between the epikarst and transmission zone on the one hand and the matrix and conduit on the other hand. However, numerical models rarely consider the unsaturated zone, let alone distinguishing its subsystems. Likewise, few models represent conduits through a second medium, and even fewer do this explicitly with discrete features. This paper focuses on the interest of hybrid models that take into account both unsaturated subsystems and discrete conduits to simulate the reservoir-scale response, especially the outlet hydrograph. In a synthetic karst aquifer model, we performed simulations for several parameter sets and showed the ability of hybrid models to simulate the overall response of complex karst aquifers. Varying parameters affect the pathway distribution and transit times, which results in a large variety of hydrograph shapes. We propose a classification of hydrographs and selected characteristics, which proves useful for analysing the results. The relationships between model parameters and hydrograph characteristics are not all linear; some of them have local extrema or threshold limits. The numerous simulations help to assess the sensitivity of hydrograph characteristics to the different parameters and, conversely, to identify the key parameters which can be manipulated to enhance the modelling of field cases.

Improving the resolution of impedance inversion in karst systems by incorporating diffraction information: A case study of Tarim Basin, China

Different scales of voids and cavities in karst systems demonstrate considerable potential as exploration targets in the Tarim Basin, northwest China. Numerous diffraction events exist in the seismic data above the karst reservoir in this area because of the strong impedance contrast and irregular shape of voids. The conventional impedance inversion method using migrated data as an input cannot easily estimate the location of voids and the impedance inside the voids. In this case, an alternative approach to impedance inversion that considers the diffractive component of the total wavefield and uses the unmigrated data as an input should be used. The inversion consists of a least-squares minimization of the misfit function between the observed and modeled data. Forward modeling incorporates a combination of reflection and diffraction wavefield components. The adopted method is applied to physical modeling and field data recorded above the karst reservoir. This study’s physical modeling test simulates observed field data and is performed using a high-resolution and high-fidelity 3D modeling system. Inversion results obtained by the proposed and conventional methods are compared. Physical modeling and a field data application show that the adopted impedance inversion method improves the karst location estimation and the acoustic impedance within the voids.

Architectural characterization of Ordovician fault-controlled paleokarst carbonate reservoirs, Tahe oilfield, China

Fault-controlled karst carbonate reservoirs are one of the most important reservoir types in the Tahe oilfield of the Tarim Basin. These reservoirs have a large oil reserve and belong to a strongly reconstructed reservoir type with a highly heterogeneous distribution of pores and fractures. This study characterizes a fault-controlled karst reservoir by using integrated methods, including outcrops, well logging, structure interpretation, seismic inversion, and statistical geomodeling. We have established a fault-/fracture-controlling karstic geologic model and classified the internal architectural elements so that we adopted an origin-controlled hierarchical geomodeling strategy based on the fault-controlling characteristics. The results determined that large strike-slip faults provide an important tectonic framework and that its derived fractures act as important channels and spaces for dissolution. Flower structure fault zones and the associated fractures are the main range of karst development, within which a high stress is concentrated during the strike-slip shear process with a high-density fracture development. This is the highly developed karst reservoir, which mainly is concentrated along large faults. The coexistence of fractures and karst dissolution has resulted in a complicated reservoir architecture (karst architecture), which can be classified into four types: (1) large caverns, (2) small caverns and vugs, (3) fractured zones, and (4) matrix (tight limestone). Controlled by the degree of dissolution, the karst architecture is quite different from the sedimentary facies. Large caverns are formed under the strongest degree of dissolution and are the most favorable reservoir type. Small caves and vugs are created under relatively strong dissolution; they are distributed outside large caves and also can act as favorable reservoirs. The fractured zones are not necessarily affected by strong dissolution but have high conductivity and act as important channels for fluid movement. The carbonate matrix is less reconstructed. The architecture development model of the fault-controlled karst carbonate reservoir presented a tree system, within which the karst reservoir caves are connected by the fractures and faults similar to fruits and trunks. The new geomodeling method revealed the constraining characteristics of faults, seismic attributes, and hierarchical architectural elements. Furthermore, we also have built a 3D model of the Tuoputai unit in the Tahe oilfield to show the robustness of this workflow. This research enables us to better understand the structure of fault-controlled karst reservoirs, and it could provide a specified characterization approach that is considered to be theoretically and practically useful.