Pressure Falloff Response Time Considerations in Subsea Wells Equipped for Through-Flowline Maintenance
Authors: Kleinhans J.W. , McAdams J.P. 
Published: December 2007

Summary
Applying subsea wells many miles from a production receiving location is feasible. In some instances, equipping wells for through-flowline (TFL) maintenance is economically attractive, particularly if well vertical reentry to perform simple downhole particularly if well vertical reentry to perform simple downhole tasks using wireline methods is costly. As the subsea well distance increases, the effect of pressure falloff response time for some types of TFL tool-string operations becomes more important. This paper describes an analytical method that can be used to predict the pressure changes with time at the remote TFL tool predict the pressure changes with time at the remote TFL tool string for a typical subsea well system. Also, analytical methods for selecting the active tool-string components, including accelerators and jars, are described.

Introduction
Subsea well applications have been used for more than 20 years to complement conventional offshore platform field developments and the more recent floating production system field developments. Subsea well applications will continue to be of increasing interest as hydrocarbon discoveries are made in deeper water and, in some instances, in unusually inclement environments. Through mid-1982, a total of 183 subsea wells had been installed worldwide, 1 and 34 of these subsea wells were equipped for TFL servicing. TFL methods can be used to minimize the need for using a drilling/workover rig to re-enter a subsea wellbore to perform relatively simple downhole work tasks. Since these rig perform relatively simple downhole work tasks. Since these rig costs can be as high as $100,000/day, the economics of equipping subsea wells for the TFL servicing from a remote producing location can be favorable. Depending on the specific subsea well configurations and number of wells, adding TFL servicing capability can increase well system costs by 10 to 25%. Although basic design methods for TFL-equipped systems have been established, TFL technology continues to be developed. Thus, each new subsea well that is equipped for TFL servicing should lead to further advances as applications experience is gained. To date, TFL methods have been applied on subsea wells that are located up to 3 miles [4.8 km] from a support location. As these methods are applied in larger line sizes and over longer distances, the effect of pressure falloff response time on TFL tool-string manipulations becomes pressure falloff response time on TFL tool-string manipulations becomes more important. Although many different types of tool-string manipulations are performed during pumpdown operations, jarring operations involve specific manipulations that activate tool-string components. These manipulations are strongly affected by the pressure falloff response time, and this paper addresses these effects. Typical TFL- equipped systems, tool strings, and a method for predicting pressure falloff response time across remote TFL tool strings are pressure falloff response time across remote TFL tool strings are discussed in the following sections.

TFL-Equipped Subsea Well
A typical subsea well system that includes TFL servicing is shown in Fig. 1. The system includes the subsea well completion, flowlines, and surface support equipment. Well completion equipment is designed to meet two basic objectives. First, completion tubulars and flowlines are selected to allow producing the well at desired rates while meeting pressure requirements. Second, TFL-related downhole equipment is selected and positioned so that TFL tool strings can be circulated to specific work points in the system. TFL equipment designs are currently available for selecting up to 20 downhole work positions. Surface support equipment is connected to the subsea well and configured to meet three objectives. First, subsea well production equipment is integrated with the pumpdown equipment. Second, pumpdown manifolding and pumping equipment is provided for pumpdown manifolding and pumping equipment is provided for circulating TFL tool strings to and from subsea-well work points while maintaining well control. Third, a TFL lubricator assembly is incorporated to allow easy and safe insertion and recovery of TFL tool strings from the pressure piping system. The high-pressure pump and manifolding system can be either portable or permanent, and pumpdown fluids may include dead oil or inhibited permanent, and pumpdown fluids may include dead oil or inhibited seawater. Also, various clear, weighted completion fluids can be used. A TFL-equipped subsea well system allows many routine and nonroutine downhole work activities to be performed from the surface support location.
JPT
P. 1082