Well Production Decline

Abstract

Effective rate-time analysis during a declining production in an oil or gas wells is an important tool for establishing a successful management.  The reasons behind the production decline include reservoir, fracture and well conditions. A well's decline rate is transient, signifying that the pressure wave propagates freely from the wellbore, leading to depletion when the outer boundary for the well is reached and to the wave propagation coming to a halt. This thesis studies the transient decline, with emphasis on a horizontal well with fracture wellbore responses. It also deals with the depletion decline, investigating the wellbore pressure responses for a vertical well producing under variable rate conditions of Arps decline. The well decline model solutions are analytical, and the modelling itself is carried out in two steps.

The first step involves modelling the transient well responses of a multifractured horizontal well. These responses originate from an infinitive reservoir and are considered as full-time rate-time responses. Multi-fractured horizontal well rate-time responses represent the solutions to a diffusion equation with varying boundary conditions and different fracture options (i.e., with or without fracture, a variety of fracture orientations, various frature lengths, etc). The transient model calculates individual fracture rates, productivity indexes and an equivalent wellbore radius for the multi-fractured well. For the transient decline of a fractured-horizontal well model, well data is matched and the servoir diagnosis and production prognosis are improved through the individual fracture production, with a model screening ability, and novel model features that can handle wellbore conditions changing from rate-to-pressure. Screening analyses can generate valuable information for fracture diagnosis in addition to a well and fracture production prognosis. Further model runs are carried out to match the real well data. The model solution is complementary to the reservoir simulation. More geology features should be considered to fully take advatage of the modelling findings.

The starting point of the second modelling step concerns late time vertical-well responses or decline curves involving empiracal solution of Arps type. This includes an investigation of well pressure responses for a rate decline of an Arps-type variable-rate of a wellbore for selected exponets, b. The modelling explores pressure wellbore responses during a variable-rate production, and the approach intorduces a no-flow specified boundary that moves outwards from a wellbore axis with a predifined speed. For the specific speed of a no-flow moving boundary, the model generates pressure profiles causing the decline exponents, b. Known b values were selected and each of them was empirically derived through the inflow performance relationships to a frive mechanism. This modelling approuch with analytically derived pressure solutions can be extended to a horizontal well. Furthermore, the continuously measured well rates and pressure models can be calibrated and verified.