Application of a New Model To Analyze Leak-Off Tests
- G. Altun (Istanbul Technical U.) | J. Langlinais (Louisiana State U.) | A.T. Bourgoyne Jr. (Bourgoyne Enterprises Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2001
- Document Type
- Journal Paper
- 108 - 116
- 2001. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 3 Production and Well Operations, 1.6.1 Drilling Operation Management, 1.14 Casing and Cementing, 1.6 Drilling Operations, 2.4.3 Sand/Solids Control, 1.7 Pressure Management, 1.1 Well Planning, 1.10 Drilling Equipment, 1.6.10 Running and Setting Casing, 1.11 Drilling Fluids and Materials
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A leak-off test (LOT), commonly known as a formation-pressure-integrity 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.
Safety concerns indicate that wellbore pressure at any depth must be kept between naturally occurring formation pore pressure and the maximum wellbore pressure that the formation can withstand without losing integrity. Knowledge of fracture pressure, which varies with depth, is as important as knowledge about formation pore-pressure variation with depth. When abnormal formation pressure is encountered, the density of the drilling fluid must be increased to maintain the overbalance to prevent possible fluid flow from permeable formations. However, there is a maximum limiting drilling-fluid density that can be tolerated to avoid fracture in the exposed shallow and weak zones below the casing shoe. This means that there is a maximum safely drillable depth into an abnormally pressured zone without running another casing string.
Fracture pressure is defined as the pressure at which an exposed formation will rupture and accept whole drilling fluid from the wellbore. Lost circulation, or lost returns, is the consequence of fractured formations. Formation fracture resistance is related directly to the weight of the formation overburden, also called the geostatic load, at a given depth of burial, the intergranular pressure of the formations, and the formation type. Thus, knowledge of formation-fracture pressure as a function of depth is an imperative requirement to plan today's deep wells in onshore and offshore environments.
Methods for determining formation-fracture pressure fall into two groups: predictive methods and verification methods. Initial well planning requires formation-fracture data based upon predictive methods, generally empirical correlations such as the Eaton correlation, the Hubbert and Willis equation, the Christman correlation, etc.1 Well-design results from predictive methods must be confirmed by a verification method while drilling a well. Because the primary objective of this study is LOT analysis, which is a verification method, predictive methods will not be covered.
The usefulness of this model lies in its ability to indicate to the engineer that an apparent fracturing (increased pump volume without pressure increase) may simply be a flow channel through the cement, and remedial operations could possibly repair the problem. It also indicates that nonlinear behavior is caused by a flow path of some sort, which may or may not warrant remedial efforts. The model does not identify the fracture point, but rather predicts the maximum attainable pressure for a nonlinear LOT, which is more a question of the magnitude of the flow path and its response to increased pressure. With the loss of drilling fluid during the test, it is obvious that a fracture has occurred, either at the casing shoe, or at a shallower depth by means of a path behind pipe. It is still an engineering judgment to decide whether the formation at the shoe actually has been fractured.
Fracture-Pressure Verification Method
Fracture pressures are verified by closing the well at the surface using a blowout preventer and pumping mud at constant rates into the closed well. This procedure is continued until a predetermined pressure value is reached or the well begins to take whole mud, indicating a significant departure from the straight-line pressure trend. The pump is stopped then, and the pressure is observed for at least 10 minutes to determine the pressure-decline rate. Because sand is weaker than shale, it is a common practice in the Gulf of Mexico (GOM) to run the test in the first sand below the casing shoe. Estimated fracture pressure from the test is used as the maximum pressure that may be imposed on the formation.
A typical LOT plot for a well with a short openhole section is shown in Fig. 1. Early test data fall on a relatively straight line, resulting from constant pressure increase for incremental drilling fluid pumped. The straight-line trend continues until Point A where the formation grains begin to lose integrity and allow mud to enter the formation. Pressure at the departure point from the straight line at Point A is the leak-off pressure (LOP) and is used to calculate the formation-fracture gradient. However, in some cases, pumping is continued until a maximum test pressure is observed. Pumping is stopped then at Point B, and the well is shut in to observe the pressure decline caused by mud or mud-filtrate loss.
Some of the main factors influencing the LOT are pre-existing cracks and faults, cement channels, plastic behavior of formations, casing expansion, test equipment, pressure gauges, injection rates, and pump efficiency.2,3 LOT behavior is examined and interpreted based on experience, but it does not provide analytical or numerical models to support these interpretations.3 It is concluded that if observed data points in a LOT depart significantly from the minimum volume line (MVL), a cement channel is suspected.3 Conversely, a computer program that predicts LOT behavior of the formations is proposed.2 However, this computer model requires several parameters that are not easily obtained. Ref. 4 presents a LOT procedure and considers the effects of mud gel strength on a LOT. It suggests obtaining this value from field-circulation data instead of a viscometer. However, this work does not consider the nonlinear LOT behavior. Wellbore compressibility is calculated along with drilling-fluid compressibility from the LOT.5 This work considers an elastic borehole-deformation effect, but not the leak effect and casing expansion. Hazov5 does not provide any model to calculate borehole-expansion volume due to elastic deformation and also does not consider nonlinear LOT behavior.
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