Coupled Subsurface and Surface model: A Case Study
Authors: Shammari B. A. , Putra S. A. , Nooruddin H.  A. , Bellaci I. J. , Shammari A. T. 
Published: March 2015

Abstract
An integrated model that couples surface and subsurface models was developed for a huge carbonate oil reservoir overlain by a large gas-cap located in the Middle East region. The main objective of the integrated model is to quickly evaluate changes in production strategy and provide more accurate forecast of field performance than with conventional approaches where surface and subsurface performance are evaluated separately. Building a fully integrated model is a very challenging task, due to the complex nature of the field process, including compositional variations, NGL processing and evaluation of gas disposition options. The surface network model was developed to allow evaluation of liquid and gas velocity in the flowlines and trunklines, and erosional velocity and back pressure to every well in the network. Trunklines were modeled with detailed elevation profiles to capture the complex nature of desert terrain found in the field. The subsurface model is a huge resolution model with more than 60 million grid-cells. The reservoir simulation model is compositional, having nine-components and runs on a state-of-the-art in-house simulator, GigaPOWERSTM. This paper highlights the process in building the fully coupled model by a multidisciplinary team, including the subsurface model, wellbore models, surface network model, and the integration layer between those different standalone models. The paper also discusses the issues encountered during building the integrated model and how those challenges were resolved.

Introduction
Integrated modeling is becoming quite popular recently in the oil and gas industry (Huang et al., 2012; Malik et al., 2012; Roadifer et al., 2012). Coupling of subsurface and surface models refer to the process of linking fluid flow equations from porous media, to the surface facilities through wellbores. During history matching process, field historical data - mainly fluid rates and pressures - are used to calibrate simulation models. The use of decoupled models at this level is enough most of the time. To predict field performance, coupled models are necessary, especially when production constrains are controlled from surface (Coats et al, 2004). Different approaches have been implemented to bring the various components of the coupled model together. Implicit coupling is a method where fluid flow equations for the surface and the subsurface are solved simultaneously at each time step. Another approach is the explicit coupling where subsurface model is solved separately and then results are passed to surface network model through a connection layer (Al-Mutairi et al., 2010).
A team was created to develop an integrated flow model to rapidly evaluate changes in production strategy, provide more accurate production forecast compared with evaluating reservoir and surface separately and to allow optimization to reduce or maintain gas-oil ratio (GOR) taking into consideration system production constraints. Due to the complex nature of the process, with planned NGL processing, frequently evaluation of gas disposition options required and the complex nature of the reservoir simulation model (compositional, dual porosity-dual permeability, etc.), the subject field was considered a suitably challenging candidate for development of an integrated model process that could be applied across other fields once demonstrated.