Value-of-Information Methods Assess Surveillance Data From Fields to LNG Plants
Authors: Carpenter Chris 
Published: September 2024

_
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 215318, “Assessing the Value of Reservoir Surveillance Data From Gas Fields Producing to LNG Plants Using Value of Information,” by Daniel I. O’Reilly, SPE, Zhi Qui Xia, and Andrew R. House, Chevron. The paper has not been peer reviewed.

_
A set of examples is shared in the complete paper for calculating the value of reservoir surveillance conducted in gas fields tying into liquefied natural gas (LNG) plants. The objective is to use existing value of information (VOI) methods to provide a means of justifying this reservoir surveillance, given that management of operating companies often is more inclined to agree to reservoir surveillance when its value is calculated clearly.

Introduction
The main objective in supplying gas to an LNG plant is to ensure that sufficient gas feed is available at all time scales (short-, medium-, and long-term). To forecast future gasfield performance, surveillance of producing reservoirs is required. The surveillance data are considered highly valuable by petroleum engineers, but a quantitative value is not always calculated.
Oil and gas engineers have considered VOI when gathering field-appraisal information for some time. Many concepts were originally borrowed from the financial industry. Two potential decisions can be identified when deciding on reservoir surveillance in producing fields: what surveillance data to obtain and, if applicable, how frequently the data should be obtained. This may influence the total cost of obtaining the data, along with its reliability.

Problem Statement
LNG plants normally are designed with a lifespan of several decades, and gas fields must supply at the plant capacity over this period. It is common for different fields to be developed sequentially over the life of the plant. It is crucial that new fields are brought online before producing fields can no longer meet plant demand.
In the medium to long term, continuous withdrawal of gas from the reservoir means that its pressure and gas deliverability decline. Initially, gasfield potential is much higher than plant demand, and often this allows for reservoir surveillance at no cost. Many operating companies favor bringing online a new field once minimum excess wellhead capacity is met (i.e., earlier than when gas supply equals plant demand). The offshore gas-rate potential normally is not directly measurable because production is constrained at the maximum rate of the LNG plant; it is a calculated value and subject to uncertainty. Well potential is calculated after the hydraulic performance of the reservoir and wellbore are fully characterized. Reservoir surveillance data are required as inputs to these calculations.
In the short term, problems may occur with the near-wellbore formation, well, tree, or subsea infrastructure that could cause reduced flow rates or require the well to be shut in. The likelihood of all production risks can be estimated using reservoir surveillance data.
Commonly, reservoir surveillance events are executed during planned or unplanned plant downtime. This avoids a situation in which plant capacity is underused because of a well being shut in for surveillance. However, many situations exist in which surveillance may require an investment or production well downtime. In these cases, an actual cost is incurred that must be evaluated. The objective of the complete paper, therefore, is to use VOI to quantify the value of reservoir surveillance data.