Most reservoir units in the Greater Cassia Gas Condensate Fields in BP acreage offshore Trinidad are usually produced by one or two wells. The permeability of these reservoirs is between 10 – 2000 mD and they are typically connected to edge or bottom aquifers of variable sizes. Water breakthrough prediction is difficult but necessary to understanding well performance and recovery from the producing wells.

Pressure transient analysis has been historically used to understand reservoir geometry and connected hydrocarbon volumes. In recent years it has been used to qualitatively describe "moving boundaries" which can be inferred from the late time analysis of the Bourdet derivative. Changes observed in this late time derivative have been interpreted to be as a result of the transient encountering a fluid boundary, such as gas/water, and the mobility contrast is detected by the pressure transient. When successive build-ups are overlain a late time derivative shift is seen.

This paper highlights the use of pressure transient analysis of build up data to help predict water breakthrough using the new Boundary Production Plot. This plot incorporates boundary distance and cumulative production to quantitatively predict when the aquifer front arrives at the producing well. It describes the methodology used in its successful application in the Greater Cassia field and its advantage over some classical reservoir engineering methods to predict water breakthrough at the producing well.

Field Overview

The Columbus Basin, offshore Trinidad, can be defined as a gravity driven extensional system that is superimposed upon a foreland basin. The dominant structural pattern in this area of the Columbus Basin is the easterly dipping NW-SE growth faults intersecting ENE trending anticlines. Where the anticlines and the normal faults intersect, the hydrocarbon accumulations occur. Most reservoirs comprise Plio-Pleistocene sand stone sediments. Reservoir sands are typically between 100 and 500 ft thick and have a wide range of permeabilities 10 to 2000 mD. Its reservoirs are sub-divided into separate reservoir units that are typically drained by one or two wells. The main impediment to gas flow is faulting. However provided some degree of sand on sand contact is maintained; faults generally do not prevent gas flow.

The Greater Cassia Field is located offshore Trinidad (Figure 1) and consists of a number of gas condensate "Fields" that flow into one major gas processing facility called the Greater Cassia Hub that links together with a number of other gas hubs that eventually flow and is sold to an onshore LNG plant.

There are many subsurface uncertainties in the Greater Cassia Field however one of the key issues is the seismic imaging issues that seriously impact the bulk rock volume uncertainty as well as the reservoir connectivity uncertainty in the heavily faulted areas. Water breakthrough prediction is one of the major uncertainties in most subsurface studies however it becomes increasingly crucial in reservoirs such as in the Greater Cassia Field where each reservoir unit is typically drained by one and sometimes two high rate gas wells that have a significant impact to the daily available gas deliverabilty. Currently there are 25 producing wells producing from 22 reservoir units in the Greater Cassia Field. The focus on this paper is on three wells in the field that have used pressure transient analysis to predict when the aquifer front arrives at the well.

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