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Developing a modular SAGD design

Process simulation models are used to optimise the design and operation of modular, small-scale bitumen extraction
RONGQI CHEN China National Offshore Oil Corporation

XIAOHUAN ZHU, LIXIA LI and KAIFENG WU, China Offshore Oil Engineering Company
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Article Summary
With increasing global demand in the energy sector, heavy and extra heavy oils and bitumen have attracted extensive attention. Vast quantities of oil sands resources have been found worldwide. The largest known reservoir of oil sands in the world is in the province of Alberta, Canada. Among these resources, over 80% of Alberta’s reservoirs are below the earth’s surface and are recovered using an in situ technique such as steam assisted gravity drainage (SAGD).

Alberta has large areas of undeveloped small scale, or spot, in situ resources in the Northern region. The challenge associated with developing these resources is the costly capex and longer payback time under current low market oil prices. These factors would represent a business risk to develop these resources. To fully exploit spot resources in a cost-effective way, the template design of a reusable and transportable Mini SAGD facility is proposed here. Compared to traditional, large scale commercial SAGD projects, the Mini SAGD facility has three characteristics: small scale production (~5000 b/d), low capital outlay (without pipeline transportation), and modularised design for transportable reuse. The overall design philosophy for the Mini SAGD facility is to provide a reusable design template for future repeatable execution which is robust and has highly operational flexibility to be suitable for different sweet pot resources with different steam to oil ratios (SOR) of 2-4. In order to support the reusable central processing facilities (CPF) strategy, the Mini SAGD design will involve varied ranges for the design basis, such as a range of SORs, changes in the diluted bitumen (dilbit) operating densities, and selected diluent densities.

As part of the design activity, it is important and useful to model and simulate the Mini SAGD processing scheme to optimise the wellpad and central processing facility (CPF) design and operation to achieve the best economic benefit. In this study, integrated, plant-wide simulation models of the Mini SAGD system with operating cost models are developed to rate the impacts of key facility parameters on operational performance. The cost impacts of spot reservoir SOR, dilbit operating densities, and diluent densities are investigated. Net cash flows based on uncontrolled market pricing of West Texas Intermediate crude (WTI) and the differential between WTI and Western Canadian Select crude (WCS) are also analysed. All of these results can provide guidelines for the process design and future operation of Mini SAGD facilities.

Process description
Fundamentally, SAGD involves the injection of high pressure steam into an underground bitumen formation and recovering the producing emulsified mixture of bitumen and steam condensate. The Mini SAGD project employs SAGD technology to produce 5000 b/d of bitumen at a typical designed SOR of 3.0 and a gas to oil ratio (GOR) of 5.0. In general, there are two proven sets of process technologies that have been employed in SAGD oil sands processing.
•    Conventional method: warm/hot lime softener + after filter + SAC + OTSGs
•    New type method: evaporator + steam generators

After screening studies, the new type technological route involving evaporator, steam generators/heat recovery steam generators (HRSG), and gas turbines was selected for the Mini SAGD project. This new process technology is suitable for a portable project and can meet the required target production capacities and dilbit quality.

A hypothetical plant in Alberta will be assumed with one wellpad and CPF that includes the oil treating unit, the water deoiling unit, the water treatment unit, the gas co-generators, the steam generation unit, and the utilities unit.  

Model description
To quantify the energy inputs and a comparative opex analysis, integrated business models with rigorous simulation models are needed to assess the operating economics of a Mini SAGD plant.

SAGD simulation model

As part of detailed data-intensive techno-economic models, rigorous SAGD operation models were built for the plant heat and material balances using the Aspen Hysys simulator (v. 10). The plant-wide SAGD simulation models consist of about seven sub-units: wellpad, oil treating, produced gas treating, deoiling, water treatment, cogeneration and steam generation, and utilities (see Figure 1).

In the wellpad, a pair of horizontal well bores is drilled into the oil sands to about 200m depth, where high pressure steam generated from the steam generators and HRSG is injected into the upper injection well, and the produced emulsion (bitumen + produced water) from the reservoir is pumped up to the CPF via the electric submergence pumps (ESP). After emulsion cooling, diluent is added to facilitate the separation of bitumen from water through the free-water knockout vessel and the treater. After cooling, the dilbit oil will be stored in product tanks for truck-out sales. The produced water flows to the deoiling unit (including a skim tank, an induced gas flotation, and an oil removal filter) for oil removal. The deoiled water is treated in the evaporator in the water treatment unit so that produced water can be reused to produce steam in the steam generation unit. A cogeneration system uses a gas turbine for the generated electricity and natural gas-fired HRSGs for additional steam generation. In the produced gas treating unit, recovered fuel gas from the produced gas and vapour recovery unit (VRU) is mixed with natural gas as the fuel supply for the steam generators.

The following are the major operating parameters affecting SAGD operating costs:
•    Dilbit densities and diluent densities (impacting diluent blending ratios)
•    Steam to oil ratios (HP steam demand)
Cost model
The major operating costs considered for bitumen production are divided into two groups: uncontrolled market pricing and controllable operating costs (opex). Uncontrolled market pricing includes WTI pricing, the WTI-WCS differential, and natural gas unit pricing. The key cost items for controllable operating costs are:
•    Dilbit truck-out and diluent truck-in fees
•    Diluent material costs (depending on the dilbit densities)
•    Variable opex:
    ν    Natural gas usage
    ν    Blowdown waste disposal costs
    ν    Chemical usages (based on required chemical dosages and oil/water flow rates)
•    Fixed opex:
    ν    Operation and maintenance fees
    ν    Manpower wages
    ν    Road maintenance and property tax
    ν    Camp fees and others

For better design and operation of the hypothetical Mini SAGD plant, the following features are incorporated into the techno-economic model developed in this work:
•    Rigorous heat and material balances from Hysys simulation models
•    Directly linking the operating parameters to financial cost models
•    Natural gas consumption: based on SOR, the thermal efficiencies of the steam generators/gas turbine/HRSG, and system energy balances
•    Evaporator blowdown costs: referenced industrial operating data of the blowdown ratios, disposal, and truck-out shipping fees
•    Chemical costs: depending on produced water qualities, chemical dosages, oil/water flow rates, and local chemical unit pricing
•    Dilbit truck-out and diluent truck-in costs and the fixed opex: referenced industrial operating data in Alberta
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