Diseño metodológico

Shock in a drilling environment is the sudden input of energy when the BHA, bit or drill string impacts the borehole. A shock peak and the number of shocks per second over a specified threshold are the shock parameters normally measured (Figure 4-39). Modern tools measure the shock in 3 dimensions so that the orientations of the impacts that cause the shocks are known.

Figure 4-39: Shocks are generally quantified by peak shock (measured in g) and the number of shocks measured over a specified threshold.

Vibration can be thought of as the cumulative energy in the shock or root-mean-square (RMS) of the drillstring response to the shock.

Shocks and vibrations can cause failure or damage to the BHA (collars, stabilizers, connections, downhole tools) and drilling bit. The potential cost impact when components in the BHA are affected due to shock and vibrations is huge. Examples of these costs may be:

• Extra rig cost due to tripping a failed BHA out of hole and running in with a new one.

• Twist off connections, leading to a fishing operation or lost-in-hole charge for the BHA.

• Over-gauge hole leading to increased mud and cement volumes.

• Inability to evaluate the reservoir due to poor hole quality and severely degraded formation evaluation measurements.

Shock and vibration measurements can be used to detect both good and bad drilling practices, and trends, which could lead to further problems. The goal is to allow drilling to continue in a safe and efficient manner. Detection, understanding and mitigation of shocks and vibrations are a way to do this.

Monitoring drilling mechanics, diagnosis of the cause of unwanted BHA behavior and subsequent mitigation of shocks and vibrations deliver improved rate of penetration (ROP) through ensuring that energy is transmitted smoothly to the cutting surfaces of the bit rather than being wasted in damaging the BHA as outlined in the following figures. This reduces the total time and cost to drill the well.

Energy In

Energy Out

Figure 4-40: Drilling Process with normal shocks and vibrations where energy is mainly used in drilling rock and hence rate of penetration is maximized.

Energy In

Energy Out

Figure 4-41: Shocks and vibrations take energy away from the drilling process thereby reducing the rate of penetration.

Shocks and vibrations are generally caused by BHA resonances due to interactions with the borehole. These may be axial, such as bit bounce; torsional, such as stick-slip; or lateral, such as forward and backward whirl (Figure 4-42). These complex interactions between the rotating drill collars and the borehole wall vary with weight-on-bit, RPM, the lubricity of the mud and flexibility of the BHA among other variables. Diagnosis of the exact cause of shocks can be difficult, however having the downhole measurements to warn of potentially destructive BHA behavior warns the driller that remedial action needs to be taken.

Figure 4-42: Various BHA resonances can cause shocks and vibrations.

5 Logging While Drilling (LWD) Fundamentals

5.1 Definition:

Logging while drilling is the measurement of formation properties during the deepening of the borehole, or shortly thereafter, through the use of measurement tools integrated into the bottomhole assembly. In comparison to wireline acquisition, LWD has the advantage of measuring formation properties before drilling fluids invade deeply, and being acquired during the drilling process so that they require minimal additional rig time. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that measurements of the subsurface are captured in the event that wireline operations are not possible.

5.2 Introduction

The conventional objective of logging, on wireline or while drilling, is to evaluate the volume, properties and producibility of any hydrocarbons in the formation. With the introduction of LWD measurements, real-time formation evaluation for the purpose of optimally placing the well in the reservoir has become possible.

Since it was first published³ in 1942, the Archie equation and its variants have been the standard method for evaluating formation water and hence hydrocarbon saturation. Gus Archie proposed the following basic form of the equation based on empirical correlation to experimental data.

n

Sw = formation water saturation a = empirically derived constant m = cementation exponent n = saturation exponent Φ = formation porosity Rw = in situ water resistivity

Rt = uninvaded formation resistivity

The classic “triple-combo” measurements (resistivity, density and neutron) are the minimum set of inputs to solve for water saturation. The density measurement is generally used to derive the formation porosity by the equation:

Bulk density,

ρ

= φ.ρ

+ (1- φ ). ρ

The neutron and density measurements are often presented on a lithology compatible scale (e.g.

limestone compatible scale) such that in a fresh-water filled formation of the selected lithology the

neutron and density measurements overlay. Any separation between the two curves is an indication that either the fluid in the pore space is not fresh water or the matrix is not that assumed to create the overlay scale. These separations help identify when the ρfluid or ρmatrix

terms in the above density-porosity equationneed to be reviewed to obtain the correct porosity.

The photoelectric factor, PEF, and its volumetric equivalent, U, which are generally available with conventional density measurement tools, add information to help evaluate complex lithologies so that the correct matrix density can be entered into the density porosity equation and hence an accurate porosity determined.

The resistivity measurements are used to evaluate the true formation resistivity, Rt. Due to the invasion of mud-filtrate into the formation during drilling, modern resistivity tools measure the formation resistivity at multiple depths of investigation to characterize and correct for the near-wellbore invasion. It has been found that for LWD measurements close to the bit the invasion is often minimal, allowing direct formation measurements and revealing differences with subsequent wireline measurements.

In addition to the formation porosity, determined using a combination of the density, neutron and photoelectric measurements; and a formation resistivity representative of the uninvaded formation resistivity; the Archie parameters, a, m and n need to be known (e.g. from core analysis) or assumed (the default values are 1, 2 and 2 respectively). The formation water resistivity, Rw, at downhole conditions also needs to be known. Rw is generally calculated based on downhole temperature and pressure, and water salinity which has been determined from water samples.

Archie’s equation can then be solved to find the proportion of the pore space filled with water, otherwise known as the water saturation, S_w. The remaining pore space is assumed to be filled with hydrocarbon. Hence, the hydrocarbon saturation, Shc = 1- Sw.

The volume of hydrocarbon per unit volume of formation is given by the porosity of the formation multiplied by the hydrocarbon saturation.

Vhc = Φ . S_h where:

Vhc = volume of hydrocarbon per unit volume of formation

Figure 5-1: Mud filtrate invasion creates an altered zone near the wellbore for which the measurements must be corrected. LWD measurements generally do not require as much correction as later wireline measurements as the invasion is not as deep soon after the hole has been drilled.

Φ = formation porosity

Shc = formation hydrocarbon saturation

Given the volume of the reservoir, usually derived from seismic interpretation along with well-to-well log correlation, the total volume of hydrocarbon in place is the volume of the reservoir multiplied by the volume of hydrocarbon per unit volume of formation. This hydrocarbon volume is generally corrected for the change in volume of the hydrocarbons as they move from downhole to surface conditions. If the hydrocarbon is oil then the volume of hydrocarbons is given as the stock tank oil in place (STOIP). If it is gas then the volume is quoted in standard cubic feet (scf) or standard cubic meters (scm).

STOIP or scf = Vhc .Vreservoir. B where:

STOIP = stock tank oil in place (volume of oil in the stock or storage tank at surface temperature and pressure)

scf = standard cubic feet (volume of gas at surface temperature and pressure) Vhc = volume of hydrocarbon per unit volume of formation

Vreservoir = reservoir volume.

B = coefficient accounting for the change in volume that occurs when the hydrocarbon moves from downhole to surface conditions.

The porosity and saturation derived from triple combo data is crucial in the evaluation of a reservoir. First in evaluating whether hydrocarbons are present, then the volume of the hydrocarbons and thus has a critical role in the decision to develop a reservoir or not.

As outlined above, the original objective of migrating logging measurements from wireline tools to drill collars was to obtain the inputs for formation saturation evaluation with minimal additional rig time, less invasion and better borehole condition than are generally present when data is acquired after the formation has been open to fluid invasion and borehole degradation.

With the introduction of Measurement While Drilling (MWD) and Logging While Drilling (LWD) tools in the late 1980’s it became apparent that the application of LWD measurements could take advantage of their unique real-time capabilities. LWD measurement utilization expanded from formation evaluation to real-time structural assessment of the well position within the geological sequence and subsequent adjustment of the drilling trajectory to place the well in the desired location.

While the formation measurements are made downhole, depth and a number of measurements derived from it are measured at surface. Unlike wireline acquisition where high telemetry bandwidth permits all the data to be transmitted and depth stamped almost instantaneously, LWD measurements are recorded downhole against time, while depth is recorded at surface against time and the time index used to merge the data to give a measurement versus depth log.