This paper examines the behavior of heavy oil reservoirs developed with horizontal and multilateral wells. Advanced decline curve analyses were used to characterize flow regimes and estimate the time to pseudosteady-state. Reservoir and well parameters such as the OOIP, Arps "b" exponent, decline rate, reserves, permeability and well productivity indices were also determined.
Example analyses are presented for single, dual and triple lateral wells from heavy oil fields located in Venezuela and Canada. All wells exhibit a characteristic extended transient linear flow regime followed by an exponential decline. Similar results were obtained whether the analyses were performed on single, dual or triple lateral wells. Interference between laterals was not observed.
Introduction
The application of horizontal and multilateral wells is gaining momentum worldwide due to their ability to drain reservoirs more effectively. This advantage is even more pronounced in tight gas or heavy oil reservoirs where low mobility is responsible for long transient flow periods. The relatively new application of these exotic well geometries to such reservoirs provides a challenge in the area of production forecasting because traditional methods and equations were developed based on flow to a vertical well.
This paper demonstrates the use of rate-time performance analyses on heavy oil reservoirs developed with horizontal and multilateral wells. Well productivity indices (PI) were calculated from the transient production period by matching the rate-time data to type curves. Permeability-thickness or the equivalent skin factor was calculated based on this PI. Hydrocarbon volume connected to the well, the Arps "b" exponent and the decline rate were calculated from the pseudosteady-state producing period.
The decline curve results were also verified using a reservoir simulation flow model. Decline curve analysis was performed on the rate versus time values generated by the flow model to confirm that the model had similar transient and depletion behavior as the actual performance data.
Decline Curve Analysis Concepts
When a well is first opened to flow, it produces under transient flow conditions. It will remain under this condition until the production from the well affects the entire drainage area. This flow condition is referred to as pseudosteady-state or boundary dominated flow. Transient rate and pressure data are used to calculate permeability-thickness and skin, whereas pseudosteady-state data are used to determine connected OOIP.
Constant well pressure solutions used to predict declining production rates as a function of time were first published in 1933 by Moore, Schilthius, and Hurst.[1] Results were presented for infinite, slightly compressible, single phase plane radial systems. The results were presented in graphical form in terms of dimensionless flow rate and dimensionless time as shown in Figure 1.
