As the petroleum reserve discovery goes deeper in the offshore oil industry, high pressure reserves are discovered. Those wells will produce at very high initial flowing wellhead pressures (> 10,000 psi) and at much lower flowing wellhead pressures after a period of production. This presents a significant challenge for design and operation of the production system, especially, the subsea production choke, being required to have very low flow coefficient (Cv) in early life to restrict flow, and very high Cv in late life to reduce pressure loss. Initially, the pressure across the choke is required to be 5000 psi or higher. Associated with the high pressure loss across the choke, the Joule-Thompson effect will cause much higher temperatures (up to 30 – 40 °F) at the downstream of the subsea choke compared to non-restricted flow. The elevated temperature challenges the materials of the downstream equipment and the pipelines/risers.
This paper presents an innovative concept ¾ using dual subsea chokes to split the pressure to protect the chokes and improve production operations. The advantages and disadvantages of using dual subsea chokes are presented.
Introduction
Major High Pressure High Temperature (HPHT) reserves are being developed in the North Sea and the Gulf of Mexico. The learning curve has been very steep on key completion technologies, subsea equipment and installations. The range of pressure and temperature are not well defined as industry consensus. In the UK sector of the North Sea the wells are called HPHT wells with a reservoir pressure (>14,500 psi or 1000 bar) and high temperature (>266°F or 130°C) [1]. In the Gulf of Mexico (GoM), operators are looking at the ultra-high pressure and temperature prospects (>25,000 psi and >450°F) [2, 3]. The current deepest well is completed at 29,860 ft TVD. New discoveries may exceed 30,000 ft. A three-tier classification has been proposed for well completion in the paper [2]:Tier I: 15,000 psi (1034 bar) and 350 °F (177 °C)Tier II: 20,000 psi (1379 bar) and 400 °F (204 °C)Tier III: 30,000 psi (2068 bar) and 500 °F (260 °C)
Fluid characterization regarding sour/sweet service is a major challenge to set the design criterion for well tabular and subsea equipment, including the choke body and trim material selection.
Figure 1 shows the subsea choking requirement on subsea choke to maintain low flowline pressures. In a 10,000 ft water depth, if a well is completed at 30,000 ft TVD, the choking pressure loss is about 5000 psi, the water depth and pipeline pressure loss is about 3500 psi and wellbore pressure loss is about 7,000 psi. The reservoir pressure loss is usually in the range of 500 -1,500 psi. Those numbers are not precise calculation results, instead, only to show the magnitudes of each component in a deepwater HPHT production system.
A reliable subsea adjustable choke is essential to a subsea production system. The critical challenge is trim material and configuration. A historic review for choke development was presented in references [4, 5]. A list of parameters for selecting a subsea production choke has been presented in reference [5] and being slightly modified and shown as Table 1. The flow characteristics of orifice-type and cage-type subsea chokes have been investigated [6].
Effective choking is critical to apply HIPP systems to the subsea pipeline [7, 8]. The choke is required to set the pressure at the inlet well below the design pressure to allow for flow transients and to provide sufficient time for HIPPS valve to close in the event of a pressure increase due to blockage.
Background
Recent discoveries of High Pressure High Temperature oil and gas reserves in the Gulf of Mexico and the North Sea presented a significant challenge to subsea production technologies, and especially for the production control. Most significantly, while the pressure differences at early production are estimated to be around 5000 psi or even higher, they are expected to substantially decrease over time. Such anticipated pressure gradient is difficult to manage in a safe and economic manner using currently known technology.
