Application of a New Model To Analyze Leak-Off Tests

Abstract

A leak-off test (LOT), commonly known as a formation-pressureintegrity test, is a verification method to estimate the fracture pressure of exposed formations. After cementing each casing string, a LOT is run to verify that the casing seat can withstand the wellbore pressure required to drill safely to the next casing setting depth. Fracture pressure determined from this test is used as the maximum pressure that may be imposed on that formation. Critical drilling decisions for subsequent casing setting depths are based on LOT results.Although a LOT is a simple and inexpensive test, its interpretation is sometimes difficult, particularly in formations that give nonlinear relationships between the pumped volume and the observed pump pressure. Ideally, a straight line is obtained that reflects the total system compressibility (i.e., the drilling fluid, the casing expansion, and the wellbore expansion.) Nonlinear LOT behavior is thought to be caused by gas in the system, by borehole failure, or by leakage of drilling fluid into the cemented casing/borehole annulus. There is, however, no mathematical model explaining nonlinear LOT behavior.In this study, a mathematical model is derived to assist in analyzing nonlinear LOT behavior. The model has been used to predict the observed nonlinear behavior of field examples. In some cases of a nonlinear LOT, the model can be used to predict the maximum fracture pressure of the formation.

Design and Analysis of Leak-Testing Methodology for Gas Lift Valves

“…The casing-integrity problem may also arise from the failure of casing shoes. To diagnose such a problem, the formation leakoff test, suggested by Wang et al (2011) and Altun et al (2001), can reveal whether the mud pressure at a certain depth exceeds the ability of the formation to contain it at the casing shoe after cementing.…”

“…Prevention of uncontrolled release of hydrocarbons forms the backbone of barrier philosophy in modern well construction and operation. Quoting Norwegian guidelines, Carlsen et al (2010) stated that "well barriers are envelopes of one or several dependent well barrier elements preventing fluids or gases from flowing unintentionally from the formation into another formation or to surface." They cited several reasons for GLV failures, including flow cutting in sealing seats caused by well unloading of completion fluids, the valve-spring fatigue failure, damage during installation, erosion during multiphase or wet gas injection, and damage to valve seat caused by chattering in a low-pressure-drop situation.…”

A New Method to Evaluate Leak-Off Tests in Shallow Marine Sediments (SMS)

“…Altun et al 2 showed that considering a small section of open hole at the time of a LOT, fluid loss to filtration for a reasonable mud system is a very small volume, particularly in the time frame of a LOT. Hence, filtration losses are not considered.…”

“…The mathematical model developed for an observed physical phenomenon in a closed wellbore system used here is mostly based on the work of Altun et al 2 The derivations for the following correlations can be found elsewhere 2 .…”

“…,…... (2) which can be written as; where; h = Open-hole length r = Open-hole radius The general solution of the total system is the summation of all subsystems' solutions. The total system solution can be written as;…”