Prospects for Horizontal & Multilateral Offshore Drilling
Boris Nikitin, Alexander Oganov, Alexander Mandel, Garri Oganov
Accelerating development of its continental shelf has been among the suggested ways for Russia to increase its crude production. In the nearest future, the waters of the Barents, Pechora, Kara, Caspian and Okhotsk seas as well as the Ob-Taz Bay are expected to become principal areas for production drilling.
However, development of the Arctic shelf’s hydrocarbon reserves (83 tcm of gas and 15 bln t of oil) will require significant investments into construction of expensive hydraulic structures, into well drilling and operation, into crude and gas transportation, as well as into personnel and production safety, protecion of the environment, etc. That is why it is extremely important to use such new trends in science and technologies that will help reduce project cost.
How to Optimize Production
Apparently, the most efficient way to drill in offshore fields is cluster directional drilling with significant borehole displacement from the vertical that allows the entire field area to be covered. Primarily, it is a wide use of extended horizontal and multilateral wells that provide considerable production growth.
In order to optimize oil and gas recovery, it is essential to develop knowledge about the structure of productive formation and fluid flow together with systems of horizontal and multilateral wells. The features of geological structure of offshore oil and gas fields should be determined on the basis of integration of the data obtained as a result of seismic surveys, geophysical and hydrodynamic investigations in the process of prospecting and exploration wells construction and probabilistic simulation of the reservoir. The efficiency of horizontal and multilateral well systems will depend on consistent and adequate application of the input information during development and implementation of the designed solutions.
Applying a rational combination of directional, horizontal and multilateral wells in the cluster method will allow a reduction the number of constructed offshore stationary platforms and structures, thus cutting down the total cost of field development, and also reaching remote offshore reservoirs. Recently, both in Russia and in the West well drilling practice extensively uses the state-of-the-art machinery, technologies, drilling and geophysical equipment and devices as well as the software to support the technological process of well construction. As a result, in a few countries there were set records in well depth, in displacement of the directional borehole and in extension of the horizontal borehole in the reservoir (Table 1).
However, records achieved in individual wells are more of a sporadic nature, not a system in today’s cluster well drilling.
It is a well known fact that, from the economics point of view, the most rational way to develop offshore fields is to employ large stationary platforms designed for drilling 24 to 60 wells or more.
To give but one example, on the basis of technical and economic studies, it was decided to develop Prirazlomnoye oilfield located in the southeast of the Pechora Sea employing a large ice-proof stationary platform (LIPSP). Initially, the plan envisaged construction of 55 wells as the basic production systems using directional wells: modified reversed seven-spot or row system with the ratio of three production wells per one injection well. The simulation results showed that the production system using horizontal wells has more promising performance. In consequence, the Field Development Plan (FDP) was amended, so as to include the drilling of 35 wells with displacements from the vertical up to 7 km and the horizontal section extension in the reservoir up to 1,000 m. Of this amount, 19 wells are producers (16 horizontal producers, two two-laterals, and one vertical well) and 16 wells are horizontal injectors.
Figs.1 and 2 show designed profiles and structures of the most extended wells that are planned to be constructed from the LIPSP “Prirazlomnaya”, using the upgraded and new drilling equipment of the Hutton unit (single- and double-lateral producers).
To develop the Shtokman gas-condensate field in the Barents Sea 92 wells would be constructed from four (three) platforms with deviation of the boreholes from the vertical about 3,000 m. The typical design profile is shown in Fig. 3.
Offshore Sakhalin, development of the Chaivo field in the northeast part of the Sea of Okhotsk has been started on land, using horizontal wells with significant borehole deviation and designed measured depth up to 10,700 m, the wells number in the cluster reaching 40.
At the same time, certain difficulties occur during cluster drilling, namely: due to different distances of the bottomholes from the platform, they differ from each other by technological peculiarities of the profiles and designs, and by other parameters.
In well construction, these factors have a negative impact on both technical-economic and qualitative showings. A new method of bottomhole placement on the field structure might become an important technical-technological solution. For example, if an oil-and-gas field is drilled by directional wells whose wellheads are located on one large cluster platform, the wells in the cluster will have different sizes of bottomholes’ deviation from the vertical, starting with the minimum value (a vertical well) and ending with the maximum one, when the bottomhole on the field structure occurs at the largest distance from its wellhead (from the point of the platform installation). It is natural that the designs, profiles and the construction technologies of these wells should be different. However, until now when planning the system of field development, this most important factor of well placement by type on the field structure, was not taken into consideration.
One of the serious problems in drilling of such fields is prevention of initiation of accidents and complications, especially an accidental meeting of the drilled and operated wells located on the cluster. In case when drilling operations on the cluster platform are carried out by one drilling rig, accidents and complications in the process of drilling may jeopardize the time of bringing wells into service and implementation of the hydrocarbon production program as a whole (for example, LIPSP in the Prirazlomnoye field).
At present, as far as the development system designing for the Russian Arctic offshore fields envisages oil (gas) production via various types of wells – from vertical to horizontal and multilateral, it should be taken into account that the efficiency of their use depends on a number of geological, technical-technological, ecological and hydrogeological conditions of well construction.
In connection to the mentioned above, an important task during cluster drilling is the correct choice of types of the wells and their placement on the field structure subject to the amount of the borehole deviation from the vertical, extension of the horizontal (flat) borehole in the reservoir, specific features of constructing wells of various type and function.
When considering the horizontal wells use as the base option in designing the field development system (Fig. 4), the length of the horizontal borehole (lhor) in the productive formation in this case is most often taken to be identical and equal to the distance between the well bottoms, prescribed according to the field development pattern, regardless of the borehole deviation from the vertical at the depth of the productive formation top (Atop). At the same time, it is natural that the more Atop is the more complex the technical and technological conditions of the well construction become and the more the degree of the risk is in carrying out the set geological and technological tasks. Hence, the wells with larger Atop and lhor have a lesser degree of reliability in terms of accident-free construction, comparatively low drilling characteristics and possibly lower production rate in operation. The conditions of well drilling changing as the Atop and total well length (Lwell) increase, require application of various types of the profile and design.
This approach envisages that horizontal wells with less borehole deviation from the vertical on the top Atop and Lwell will have the largest value of lhor, and as Atop increases, the lhor value reduces, i.e. as the well drilling conditions become more complicated, the length lhor, being the most complicated interval of the horizontal well drilling, decreases. Hence, the wells which are most remote from the stationary offshore platform and which have maximum Atop values, will be drilled with a less value of lhor and in the “marginal” case they will be not horizontal, but flat wells crossing the productive formation at a constant inclination angle, equal by its value to the angle at the depth of the productive formation top.
So, this approach enables to select lhor differentially, taking into account providing a more reliable technical safety, preserving for the entire field the calculated value of the total horizontal borehole extension in the productive formation. Moreover, the horizontal wells with a larger lhor value will be located in the vicinity of the platform, which as a rule is placed in the part of the field where the highest production rate is planned, which in its turn will provide significant increase of the total well production rate. The wells which have the least deviation from the vertical on the top of the productive formation Atop should be drilled in the first place.
Construction of multilateral wells (MLW) especially with radial branches coming from the main horizontal borehole or a flat borehole, in our opinion, may be assumed as a basis of the development systems for a number of the Russian offshore fields, including those in the waters of the Ob-Taz bay, in the near future. Therefore the world experience in designing and constructing laterals from the main borehole presents a certain interest.
The main areas of operations in MLW construction are the Gulf of Mexico, the U.S. West Coast, South America, the North Sea, the north slope of Alaska, Southeast Asia, Italy, and the Middle East.
The plan of MLW drilling should envisage the geological factor of the area of expected drilling, adequate provision of equipment for drilling and operation and other essential materials.
As is well known, the main parameters interaction of which minimizes occurrence of possible problems in MLW designing and which enable to evaluate successfulness of a MLW making are well life time, well type, i.e. its level according to the international TAML classification, borehole profile, casing string structure, cementing program, and if required, the program, of reservoir perforation and the type of equipment applied for completion.
The most important aspect of providing a high-quality MLW construction is application of technical and technological solutions on designing efficient profiles and programs of multilateral wells. A standard design profile of a two-lateral well with a flat and horizontal borehole branches from the vertical, calculated by the well-known technique of the authors, as well as a diagram of laterals of a radially horizontal well are given in Figs. 5 and 6.
As everybody knows, in case of bottom water absence, hydrocarbons production grows to the maximum if the borehole is drilled in the central part of the productive formation and decreases as the borehole reaches the top or the bottom of the productive formation. An important factor influencing the efficiency of construction and production of horizontal and multilateral wells is also the shape of the borehole path within the reservoir. Fig. 7 shows a diagram of the three most common types of profiles of the end part of a horizontal/multilateral well. The first type consists of four intervals: gathering inclination angle a up to 90° within the reservoir, stabilization of inclination angle a, buildup of > 90° and decrease of the inclination angle to the minimum required amin. The second type profile is a three-interval one (“ascending”), which is derived from the first one by exclusion of the last interval of the inclination angle decrease. The third type profile is a three-interval one (“descending”), in which, in contrast to the second type profile, instead of the interval of curvature buildup of inclination angle a > 90°, the interval of the inclination angle decrease to the minimum required amin is envisaged. The third type profile is more preferable, as drilling according to the first two profiles is more complicated because of the difficulties related to additional axial loads and frictional forces originating during round-trips. Moreover, the presence of a concave part in the end section of the second type profile can contribute to formation water accumulation which in its turn hampers oil movement and results in a premature drowning of the well.
At present, it is possible to claim that for the above types of profiles of the end part in the reservoir, the effective ratios of the intervals lengths for a three-interval profile are the following: l1 @ 0,1Lh; l2 @ 0,6Lh; l3 @ 0,3Lh, for the purpose of increasing the overhaul (“non-watered-out”) period of well operation, as well as based on technological considerations and taking into account foreign investigations.
The MLW drilling successfulness is based, in particular, on recommendations for assessment of well completion systems, including the following considerations: necessity of the laterals shutoff, selection of the type of equipment and technological measures for well operation life increase, compiling of the program for management of the inflow from the reservoir, possibility of workover jobs under applied equipment for completion and operation of MLW’s, possibility to adjust the parameters of the completion system.
After selection and assessment of the completion systems for construction of a specific MLW it is possible to determine the method and techniques of drilling.
The process equipment, the technology and methods of stabilization of the laterals are new challenges in horizontal drilling and require a careful calculation in designing a MLW.
Provision of high-quality implementation of the elaborated stabilization program depends on the correct solution selection for each of the above issues affecting the cost of the MLW via the cost of the applied equipment, materials and the implemented stabilization operations proper. The wrong solution in designing often leads to risk increase during construction of a specific MLW. It is important to emphasize that there is a close interrelation between the drilling technology of such complex wells and economics, lying in the fact that a delay of the time of well construction termination results in decrease of the field development rate, and consequently to deficiency of compensatory expenses and oil and gas sales income sights.
At the same time it should be noted that the modern stage of the horizontal and multilateral drilling development is characterized by transition from the practice of sinking individual wells to the system of oil and gas fields development by such wells (for example, offshore fields in the North Sea, such as Western Sleipner and Troll, in the Sea of Okhotsk and on the shelf of Argentina).
Therefore one of the strategic tasks in development of the shelf fields of the Arctic seas in Russia, in our opinion, is extensive application of horizontal and multilateral drilling, which will allow a significant decrease of the number of wells for efficient development of the fields. Consequently, in its turn, the fields development with a minimum quantity of platforms might make a number of offshore fields located in severe natural-climatic environment attractive for their most rapid development.
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