Topics in Production Systems Modeling: Separation, Pumping and Model Based Optimization

Abstract

This thesis addresses three distinct topics within oilfield production technology: 1) Inline oil-water separation for subsea applications, 2) Model based constrained optimization for production networks of high water cut wells boosted by ESPs (Downhole Electric Submersible Pumps), and 3) Hydraulic analysis of a novel configured hexagonal positive displacement pump.

While each of the three topics in the thesis is investigated and discussed in a stand-alone manner, they all share a common industry objective; increasing the yield and prolonging the viable production period of hydrocarbon producing fields. More specifically, they reside within two important classes of production technology challenges; (a) boosting the deliverability and the flow of wells with high water content, and (b) separating and removing of water from hydrocarbon streams as close as possible to the source in a production gathering system. Numerical modeling is the main methodology employed in the three topics, where modeling results are substantiated by field scale or laboratory generated data.

The inline oil-water separation technology addressed in this thesis is based on a controlled and distributed tapping from the lower side of a water rich stream flowing in an inclined pipe spool. The long term objective is to develop a capability for seabed separation near the subsea wells in mature offshore fields with high water production and declining reservoir pressure. The intention is to reduce the backpressure on the wells and increase or maintain their production level. The production gain is achieved by harnessing and hydraulically manipulating the energy of the inlet mixture stream to reduce the backpressure exerted by the outlet streams. Important and unique features of the concept are; the separation and phase splitting do not consume external energy, there are no major moving parts, and there is inherent performance tolerance to deviations from the design set-points.

The thesis expands an earlier IPT/NTNU concept verification research project (Sponsored by the Research Council program DEMO 2000) which involved experimenting with a low pressure full scale separator test facility. This thesis progresses the relevant previous knowledge and information from a concept validation level to establishing and validating a more detailed design strategy and a more focused performance design for the separator. The thesis brings the investigated separation approach to a mature level where the fluid mechanics design aspects are largely clear and understood and are ready as an input for the mechanical design of a separator prototype.

The separation was analyzed from the multiphase hydraulic design point of view using numerical experimentation as the primary tool. The research methodology comprised of conducting the following tasks: (a) developing a procedure to assess the potential production gain of installing the inline separator in a subsea production system and to identify the design requirements for obtaining a specified separator performance, (b) introducing and demonstrating concepts to quantify the drainage performance of a single and multiple taping points, (c) Validating the usefulness of 3D CFD (Computational Fluid Dynamics) methods to represent  the fluid dynamics details of an oil-in-water dispersion and separation, (d) Employing the same 3D CFD model to reproduce the laboratory experimental results.

The other two topics in the thesis constitute a response to emerging field scale problems where the industry have called for an immediate and sound modeling based diagnostic and modeling based investigative design.

The second topic addresses an optimization strategy for large oil production systems consisting of clusters of high water cut, low GOR oil wells producing by ESP. The production streams of the wells converge through a multi branched surface gathering system into a system of main flow conduits leading to a single processing plant. The objective is to perform a model based numerical optimization to maximize oil production and reduce lift costs by modifying ESP rotor rotation frequency while complying with multiple operational constraints. While industry is currently in possession of tools to perform such tasks the outcome is inconsistent and yields poor optimization result when modeling large system with many wells, complex network and large number of constraints. An investigative task to clarify the source of the difficulties was deemed necessary.

The optimization technique is described in the thesis and employed to quantifying the achievable production gains. It also identifies the computational hurdles encountered in computing the global production optimum. The thesis reports and discusses modeling and optimization using three cases: two are scaled-down synthetic cases to establish the fundamentals of the computational process, and one case on a field-scale production system is used to capture the impact of system complexity. The observed outcome and the conclusions of the investigation provide bases for a robust and consistent production optimization program of a large field. The details of this industrial scale project are beyond the scope of this thesis

The third topic deals with modeling and critical analysis of a novel design of a positive displacement pump for drilling mud circulation. The concept has been commercialized and launched to the offshore market in recent years (commercially called “Hex pump”). The obvious attractiveness of the pump is its compactness and its small footprint when mounted on congested offshore platforms. However, the pumping performance of the pilot installation was very poor exhibiting excessive pulsation, vibration, mechanical failures and noise. These have driven expensive and critical drilling operations offshore to a halt. It has been recognized at this stage that the unique and innovative design features of the pump together with the criticality of it good and safe performance warned a thorough model based concept analysis and verification. The thesis describes the hydraulic performance modeling and its use to identify the concept inherent pulsation generating source. The conducted modeling and its interpretation are of novel nature and the results revealed a fundamental conceptual flaw. The research outcome had a prompt and an immediate impact on the industry decision of deploying this novel pump type.