Wednesday, March 14, 2018

Measurements at the Bit: MWD Tools

Measurements-while-drilling technology has moved down the drillstring to enlist the bit itself as a sensor. 

Conventional drilling of high-angle and horizontal wells is like piloting an airplane from the tail rather than the cockpit. Information required to land the well in the target formation is derived from sensors 50 ft or more behind the bit or at the surface. Because these measurements -about well trajectory, drilling efficiency and formation properties -are remote from the bit, crucial drilling decisions are delayed and data may require more complex interpretation. In particular, course corrections are delayed by lag in measurements needed to make steering decisions, resulting in less drainhole in the pay zone. Also, maximum drilling efficiency requires information about mechanical power delivered to the bit, which is inferred from surface measurements, degrading its accuracy. And resistivity measurements from logging-while-drilling (LWD) sensors in drill collars are limited to formation resistivity less than 200 ohm-m.

Despite these limitations, horizontal and high-angle drilling have proved successful, especially in simple geologic settings - uncomplicated layer-cake structure. Nearly, all these  wells start vertically, with a conventional rotary bottomhole assembly (BHA). The drillstring and bit are rotated from the surface either by a rotary table on the derrick floor or a motor in the traveling block, called a topdrive. Drilling this way is called rotary mode. To kick-off from vertical, the rotary assembly is replaced with a steerable motor - usually a positive displacement motor, driven by mud flow, in a housing bent 1 degree to 3 degree. When mud is flowing , the motor rotates the bit, but not the drillstring. This type of drilling is called sliding mode, because the drillstring slides along after the bit, which advances in the direction of the housing below the bend. 

The direction in which the bit is pointing, called toolface, is measured and sent to surface by measurement-while-drilling (MWD) equipment for real-time control of bit orientation. Measurements include azimuth, which is the compass bearing of the bit, and inclination, which is the angle of the bit with respect to vertical. Large changes in direction are made by lifting off bottom and reorienting the bent sub by rotating from surface. Small changes are made by varying weight on bit, which changes the reactive torque of the motor and hece toolface orientation.

Once sufficient inclination has been built, straight or tangent sections can be drilled in several ways. One is with a conventional rotary, or "locked" , assembly, which is rigid enough to allow fast, straight drilling. Small adjustments in inclination can be made by varying weight on bit or rotary speed. Most horizontal sections, and some tangent sections, are drilled with a steerable motor while rotating the drillstring from surface. In this mode, the steerable motor behaves like a rotary BHA, maintaining both azimuth and inclination. 

However, the presence of the steerable motor allows the driller to make course corrections without tripping the drillstring out of the hole. 

Generally, the driller tries to make as much hole as possible using a rotary assembly or a steerable motor in rotary mode. Rotation of the drillstring reduces the risk of getting stuck and allows faster drilling than in sliding mode.

Overcoming limitations in horizontal drilling

Today, the ability to drill horizontally is undisputed. Yet, the efficiency of drilling and steering horizontally is limited by the distance between the bit and measurements. In drilling, for example, one way to define efficiency is the ratio of time spent making hole to the total rig time, including operations such as trips or hole conditioning. In the horizontal section, steering efficiency can be defined as the ratio of the length of the horizontal section in the pay zone to the total length of the horizontal section. How does lag between measurements and the bit limit these efficiencies?

In drilling with a downhole motor in rotary mode, a key limitation on efficiency is how much weight the driller can safely apply to steerable motor. As the driller increases weight , the motor produces more torque, and power is torque times RPM. The more power, the faster the rate of penetration -up to a point. Excess weight may stall and eventually damage the motor, requiring an expensive trip for motor replacement. The goal is to apply as much power as possible, but within the operational limit of the motor. Power is estimated conventionally from surface measurements of mud flow and mud pressure. Motor RPM is roughly proportional to mud flow. Torque is roughly proportional to the increase in the mud pressure when the bit is on the bottom, compared to off bottom. 

Perhaps the greatest limitation in conventional horizontal drilling is in steering efficiency. Wells are conventionally steered "geometrically" - along a path that has been predetermined based on nearby well data and geologic assumptions. Steering is based only on bit direction and inclination data. Gamma ray and resistivity measurements, if present, are made far from the bit and used only retrospectively. This technique is fine, as long as the target is thick, structurally simple and well known. But it is less effective when the target is thin, complex or insufficently known for planning the well trajectory. And increasingly, with advances in three-dimensional seismics, operators are locating more intricate reservoirs and drilling more complex wells. Challenges today include thin beds and complexly folded or faulted reservoirs.

In these settings, sensors in drill collars allow replacement of basic geometric steering with more efficient geologic steering, or "geosteering" - navigation of the bit using real-time information about rock and fluid properties. A North Sea example shows how LWD sensors performed the dual purpose of geosteering and formation evaluation. Using mostly resistivity measurements, the driller geosteered a drainhole along the top of the oil/water contact to avoid gas production. Resistivity modeling from offset wells showed this contact should have a resistivity of about 0.6 ohm-m. When the value dropped, indicating water , the well path was turned up slightly; when resisitivity increased, the well path was dropped slightly.  

In addition to reduced efficiency in drilling and geosteering, a third limitation of conventional horizontal drilling is in formation evaluation while drilling. Logging-while-drilling sensors reach the formation long before wireline measurements, and so generally view it before wellbore degradation, but some invasion has still occured. Rapid invasion, called spurt, may mask true resistivity in some formations. Also, LWD resistivity measurements by the CDR Compensated Dual Resistivity tool are limited to environments favoring induction-type settings -resistive mud (fresh or oil-base mud) and conductive rock.

The solution to these problems -limited efficiency in drilling and geosteering, and limited capabilities of real-time formation evaluation - is relocation of drilling and logging measurement to the bit itself. The system includes two new logging devices : the Geosteering tool , an instrumented steerable downhole mtoor and the RAB Resisitivy-At-the-bit tool, an instrumented stabilizer. Measurements include gamma ray, several types of resistivity including a measurement at the bit, and drilling data such as inclination, bit shocks and motor RPM. 

The technical leap that allows measurements to be made at the bit and below the steerable motor is a wireless telemetry system. This telemetry link sends data from sensors near the bit to the MWD tool up to 200 ft behind the bit, a path that bypasses the intervening drilling tools, such as the steerable motor. The PowerPulse MWD system recodes and then sends data to surface in real time using mud-pulse telemetry at up to 10 bits per second. At surface, data recording, interpretation and tool control are performed by the Wellsite Information System. Control data can be sent from the surface back downhole by varying mud pump flow. 

The geosteering tool enables the driller and geologist to make real-time correction at the bit, detect hydrocarbons at the bit and steer the borehole for increased reservoir exposure. Both tools measure gamma ray, resistivity using the bit as electrode, and "azimuthal" resisitivy - focused at a narrow angle along the borehole wall. 

Resistivity at the bit is measured by attaching the Geosteering or RAB tool directly to the bit and driving an alternating electric current down the collar, out through the bit and into the formation. The current returns to the drillpipe and drill collars above the transmitter. In water-base mud, returning current is conducted from the bit through the mud, into the formation and back to the BHA. In oil-based mud, which is an insulator, current returns through the inevitable but intermittent contact of the collars and stabilizers with the borehole wall, leading to a qualitative indication of resistivity. Formation resisitivy is obtained by measuring the amount of current flowing into the formation from the bit, and normalizing it to the transmitter voltage. 

Azimuthal resistivity is measured from one or more button electrodes and , like the azimuthal gamma ray measurement, can be used to steer the bit. Both tools can be oriented in multiple directions to find the location of a lithologic or pore fluid boundary relative to the borehole -up, down, left or right - and thereby steer the bit. 

Surface Control for Measurements at the Bit

Because the Geosteering tool is an instrumented steerable motor, it enables the driller to steer the bit on a geometric or geologic path through the pay zone.  The driller's window into the bit is the Wellsite Information System, which includes a display for checking and revising the structural and stratigraphic model, and updating the drilling trajectory. This screen is intended mainly for real-time management of horizontal drilling. 

 Resolution of both Geosteering tool and RAB Resistivity measurements is sufficient for hydrocarbon detection and lithologic correlation. The multiple depths of investigation and high resolution of the focused RAB measurements also provide formation evaluation-quality information. Applications include prompt location of coring and casing points, and monitoring of invasion by logging after drilling.

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