Wellhead gas compression maximizes gas feed to Pakistan gas plant

April 4, 2011
The Bhit compression project is a response to the natural depletion of the Bhit gas reservoir north of Karachi in Pakistan and will maximize well productivity as reservoir pressure declines.

C. Corden
Costain Energy & Process
Manchester, UK

The Bhit compression project is a response to the natural depletion of the Bhit gas reservoir north of Karachi in Pakistan and will maximize well productivity as reservoir pressure declines.

Eni Pakistan Ltd., operator of the Bhit gas plant (BGP) and field, engaged Costain Energy & Process for the engineering and procurement services for the project.

The project's first gas milestone was installation of a 3.3-Mw compressor package on the Bhit 11 wellhead about 6 weeks earlier than scheduled (Fig. 1).

Construction is in progress on the compression installation next to the Bhit 11 wellhead (Fig. 1).

The project team now is finishing a 24-month construction and installation program to supply similar self-sufficient compression at 10 gas wellheads.

At the end of February, six of the wellsites are operational with the final one scheduled for installation in early 2011.

Eni Pakistan has a 40% interest in Bhit. Its partners are KPBV (Kirthar Pakistan BV-Shell) 28%, OGDCL 20%, PKP 1 Kirthar BV (Premier Pakistan) 6%, and PKP 2 Kirthar BV (Kufpec Pakistan) 6%.

Project background

The Bhit gas reservoir and processing plant are in a remote, rural location in Sindh province, Pakistan (Fig. 2). The ten producing wells are spread over a large area on top of the Bhit Mountain, at elevations of up to 850 m. All wells connect to the main cross-country gas gathering main—a 21 km, 24-in. OD pipeline to the BGP.

The Bhit gas processing plant is in Sindh province of Pakistan (Fig. 2)

The BGP processing trains include water and condensate removal, cryogenic nitrogen removal, and export gas compression. The product gas is exported into the Sui Southern Gas Co. (SSGC) pipeline, where it constitutes up to a third of the total natural gas supply to Karachi.

The minimum operating inlet pressure at the BGP is 730 psig, dictated by the required nitrogen removal unit (NRU) performance. At full flow, this corresponds to a pressure of about 800-850 psia at the wellheads, depending on their locations relative to the plant. When the Bhit gas plant was designed in 2000, the wells easily achieved this flowing pressure without compression.

Reservoir simulations predicted that declining reservoir pressures and hydraulic limitations imposed by the gathering system pipework would start to affect plant throughput by mid 2009. In anticipation of this, the operator started the Bhit compression project concept study at yearend 2005 to consider options for the provision of feed-gas compression.

Concept report

The concept study considered three options for the front-end compression facilities:

1. Wellhead compression.

2. Intermediate compression (within the gas gathering system).

3. Plant compression (within the existing gas treatment plant).

The study specified the compressor configuration for each option to meet flow capacity, reliability, and operating flexibility requirements.

One of the reciprocating compressors, driven by gas engines, is undergoing a factory test (Fig. 3).

At individual wellsites (Option 1), the maximum power requirement proved to be within the range of reciprocating compressors, driven by gas engines (Fig. 3). This offered a reduction in capital cost and infrastructure when compared to a similar centrifugal machine. For Option 1, management of wellhead flows across the network, with no requirement for duty/standby machines at each site, could meet all overall reliability targets.

Options 2 and 3 would require multiple centrifugal machines and gas turbine drivers..

A qualitative selection process eliminated Option 3 on the basis of cost and constructability, given the extensive on site work required.

The operator selected wellhead compression (Option 1) following technical and economic analysis and the following features:

• Favorable economics, due to increased total gas recovery.

• Significant safety advantage, identified by a hazard identification (HAZID) review that compared diverse operations to the common facilities proposed in other options.

• The possibility of phased, time-managed installation according to individual wellhead requirements.

• High reliability, demonstrated to have minimal impact on the overall reliability of the facilities.

• Maximum flexibility of wellhead operations.

• The option to decommission individual compressors as wells become redundant.

• Increased potential for reuse of the compressors at new wellsites.

Compressor duty

The total amount of gas recovered from the reservoir is critical to the economics of the scheme. The selected abandonment pressure at the wellhead fixes the minimum compressor-suction pressure and must be set by optimizing capital cost against total gas recovery.

The concept study specified that the compressors should maintain a production rate equal to 300 MMscfd of sales gas for an operating suction pressure range from 850 psia (start of life) down to 250 psia (end of life). A single stage of compression could accommodate this range.

As the project progressed to detailed design, further economic analysis against the reservoir models changed the compressor specification. The new specification reduced the minimum suction pressure to 100 psia. This pressure allows for higher production rates for the remaining life of the field, once plateau production (to 250 psia) is complete.

Reconfiguring the compressor so that the cylinders operate in series rather than in parallel when the suction pressure falls will allow for the lower suction pressure. No design change was made to the total installed power of the compressors.

The respecification of the compressors introduced another vessel and increased cooler area at each site and also had a small effect on the ancillary equipment. The predicted increase in total gas recovery from the field, however, easily justified the additional cost.

Wellhead stream processing

The BGP produces a clean, high calorific value sales gas for export, but the field also requires some wellhead processing to condition the gas for compression, for use as fuel gas in the gas engines, and for transfer to the main plant.

The wellhead stream contains free water that must be removed upstream of the compression stage. Because there is no water treatment at the individual wellheads, the current operation reinjects this water into the compressor discharge stream that enters the processing plant for treatment and disposal.

At concept stage, the design team considered a system of blowcase vessels in which gas from the compressor discharge pushes liquid out into the downstream pipework, without the requirement for pumps. For this application, however, the detailed design review identified several technical risks and the installed system uses a conventional flash vessel and pump to give high reliability and good operating stability.

Local fuel-gas conditioning at each wellhead provides a gas that meets the minimum specifications required by the gas engines in the power generation and compressor packages. This includes a filter coalescer to remove particulates and liquids and an electric heater to heat the gas well above its dewpoint.

The gas in the Bhit field contains 19% nitrogen (molar basis) and therefore has a lower calorific value than most industrial fuel gases. During the design process, the use of membranes was investigated to reduce the level of inert nitrogen, but the installed units are currently operating without this additional pretreatment.

New wellhead piping is made of a duplex stainless steel, but the compressor package and existing gathering system are carbon steel.

At each wellhead is a corrosion-inhibitor package to protect against the CO2 content (1.1 mol %) and free water in the gas stream. The design uses NACE standards to allow for a potential H2S content of up to 2 mol % later in the life of the field.

The compressor installation includes modular skid mounted units (Fig. 4).

Fig. 4 shows a completed compressor installation and Fig. 5 shows one of the wellheads.

The Bhit wellheads sit atop Bhit Mountain, at elevations up to 850 m (Fig. 5).

Design requirements

The use of standardized design elements across the different wellheads already has provided benefits in cost, program, and quality-assurance requirements throughout the engineering and construction phases. This standardization will also assist operating and maintenance teams throughout the life of the plant.

Based on maximum reservoir production potential, the project specifies two compressor package sizes for the 10 wells, (4 × 3.3 Mw and 6 × 1.9 Mw). A single discharge pressure of 850 psi has been set, based on the individual well with the highest outlet-pressure requirement.

The produced-water equipment also is standardized and designated high rate and low rate, according to predicted flows at each wellhead. The fuel-gas conditioning units are common to all sites.

The units are modular and skid mounted wherever possible to minimize field construction requirements. Each package layout is identical within the skids, although they have variations in orientation and tie in locations to allow safe integration with existing equipment and to minimize the requirement for extension to the individual site boundaries.

Instrumentation and cabling installation also are standardized as much as possible.

Each site has several power sources available:

• The compressor is driven by an on-board gas engine.

• Power generation for other package items is by a second gas engine, standardized across all sites.

• A battery-powered uninterruptible power supply will provide up to 24-hr power to the control systems in the event of a gas engine trip, allowing operators sufficient time to travel to the more remote sites.

• A single mobile diesel generator provides a back-up power supply for the wellhead facilities in the event of the unavailability of the normal power generation facilities. This generator can be mounted on a trailer for transport between sites.

• Two smaller black-start diesel generators are available for start-up of the wellsite facilities following a shut down. These units have wheels and can be towed to the required locations.

Health, safety, environment

Eni has a formalized health, safety, and environmental (HSE) policy protection and a site-specific HSE management plan.

A key goal of the work was application of best practice in terms of HSE standards and Costain has executed all engineering activities in accordance with common international norms.

The Constitution of Pakistan assigns mineral oil and natural gas production to the control of federal government. In addition to these federal laws, the facility is also subject to provincial laws governing environmental pollution and the protection of labor, wildlife, and cultural heritage.

Although the preparation and submission of a safety case is not a current legal requirement within Pakistan, Eni has committed to completing a plant safety case of a standard comparable to that required under the UK control of major accident hazards (COMAH) regulations.

The existing BGP safety case has been fully updated for the gas compression project following a quantitative risk analysis (QRA) to assess the major accident hazards and major accidents to the environment associated with the operation of the new wellhead facilities.

Early in the project, an environmental hazard identification (ENVID) review was carried out and the design included a detailed environmental impact assessment and noise study.

As for previous phases in the Bhit operations, the project uses principles of mitigation by design and the application of best available technology to reduce or eliminate many environmental hazards.

The Bhit compression project can be considered as a minor extension of existing industrial activities across the Bhit field. As such, the effect on the project area's natural and socioeconomic environment is low and in compliance with applicable environmental and social regulations.

All identified environmental sensitivities and risks, such as minor increases in wellhead emissions and the provision of facilities for the construction and operating work force, are carried forward in the plant's environmental management plan. This plan proposes mitigation and monitoring measures to remove or reduce negative effects as far as possible during the full course of the activities on the Bhit lease.

The author

Caroline Corden is a principal process engineer with Costain Energy and Process in Manchester, England. She has more than 15 years' experience with the company, working predominantly in cryogenic separation processes, industrial gas purification, and natural gas processing. Corden holds a BEng in chemical engineering from the University of Birmingham and is a chartered member of the Institution of Chemical Engineers.
CORRECTION

At left is the corrected Fig. 2 for the article "Hammer bits, underbalanced systems optimize Canyon sand drilling," by Deepak M. Gala, Greg Hawley, and Craig Lagrandeur (OGJ, Feb. 7, 2011, p. 73).

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