Method of Predicting Rock Strength Using Drilling Data for Horizontal Well Placement

The present disclosure relates generally to geosteering a wellbore using predicted confined compressive strength (CCS) and unconfined compressive strength (UCS) of subterranean formations.

Slow drilling is a common challenge while drilling tight subterranean formations. ROP (rate of penetration) is predominantly a function of rock strength, and tends to be higher if rock strength is lower and lower if rock strength is higher. In addition to rock strength, ROP is also related to drilling parameters such as weight on bit (WOB), revolutions per minute (RPM), torque, drill bit design, bit wear conditions, formation pore pressure, mud weight and other mud properties. Subterranean formations are typically heterogeneous in both horizontal and vertical directions, which makes it a very challenging task to predict the variation of rock strength before drilling. Without the knowledge of rock strength of the rock being drilled, it is difficult to diagnose why the ROP is low and what actions should be taken to improve the ROP.

If rock strength can be evaluated while drilling, it will greatly help to geo-steer the well to the lower rock strength regions for improved ROP. By placing the horizontal wellbore in the lower rock strength layers, it helps to ensure a better drilling performance. The lower rock strength rock is also typically rock with better porosity and permeability and therefore better reservoir quality. Since rock strength is very well correlated with reservoir quality in most reservoir formations, placing the wellbore in lower rock strength rocks also ensures the wellbore to be in better contact with higher quality reservoir rock.

Rock strength can be measured on core plugs with unconfined compression tests or triaxial compression tests. The measured rock strength can be either UCS (unconfined compressive strength) or CCS (confined compressive strength) at specific confining pressures. CCS is higher at higher confining pressures and CCS is higher than UCS in general. Rock strength can also be calculated using sonic, density or other types of logs and the calculated rock strength is typically calibrated with rock strength measurement from core testing results.

While drilling a well, rock strength can be evaluated with logs from Logging-While-Drilling (LWD). However, LWD logs are not always available. Even when they are available, the LWD tools are at some distance behind the bit and therefore the evaluated rock strength is delayed and can be available hours after the formation is drilled.

In the field of drilling wellbores in subterranean formations, there is a need for predicting rock strength using downhole-measured drilling data while drilling, which helps in geosteering to place the wellbore in the most favorable reservoir formation layers and improving rate of penetration (ROP).

Various details of the present disclosure are hereinafter summarized to provide a basic understanding. This summary is not an exhaustive overview of the disclosure and is neither intended to identify certain elements of the disclosure, nor to delineate the scope thereof. Rather, the primary purpose of this summary is to present some concepts of the disclosure in a simplified form prior to the more detailed description that is presented hereinafter.

In certain embodiments, confined compressive strength and unconfined compressive strength of subterranean formations are predicted using for example downhole drilling parameters measurement, equivalent circulating density, mud flow rate and drill bit data. The predicted unconfined compressive strength helps to geo-steer the wellbore, which may be horizontal, in the most favorable reservoir formation layers. In certain embodiments, predicting rock strength using downhole drilling data in real-time for geo-steering horizontal wells in the most favorable formation layers is disclosed. Monitoring rock strength while drilling for improving rate of penetration can be performed. In certain embodiments, monitoring rock strength while drilling for assisting the placement of horizontal wellbore in the formations with lower rock strength is performed. In certain embodiments, developing a real-time monitoring workflow that can be applied to monitor the properties of formations being drilled is performed.

In certain embodiments, rock strength from drilling parameters which are available as soon as the formation is drilled is predicted. The predicted rock strength therefore provides much earlier information for making timely decisions.

Since there is always difference between surface drilling parameters and downhole-measured ones, and downhole-measured parameters are the ones better related to rock strength, downhole-measured drilling parameters are therefore used in certain embodiments.

In certain embodiments, the confined compressive strength (CCS) and unconfined compressive strength (UCS) of subterranean formations is predicted while drilling horizontal wellbores. The monitoring of the variation of UCS thus helps to place the horizontal wellbore in the most favorable reservoir formation layers.

In certain embodiments, a method that predicts UCS of subterranean formations using real-time data while drilling a horizontal wellbore is disclosed, and with such UCS, the method assists in drilling performance improvement and optimization of horizontal wellbore contact with reservoir formations.

According to an embodiment consistent with the present disclosure, a system for predicting rock strength for use in horizontal well drilling includes a memory to store computer executable instructions and one or more processors, operatively coupled to the memory, that execute the computer executable instructions to implement a rock strength analyzer configured to determine unconfined compressive strength (UCS) from input parameters, and a well placement module operable to monitor placement of a horizontal well bore in a drilling operation based on the CCS and determined UCS. In certain embodiments, the rock strength analyzer can include a specific energy module for determining specific energy from mechanical and hydraulic specific energy, and a compressive strength module for determining the UCS from confined compressive strength (CCS) as related to specific energy by a mechanical efficiency E.

In another embodiment, a computer-implemented method for predicting rock strength for use in horizontal well drilling includes using drilling and bit data to calculate mechanical specific energy (MSE), using drilling, mud and bit data to calculate hydraulic specific energy (HSE), obtaining a specific energy value from the MSE and HSE, calculating confined compressive strength (CCS) from the specific energy, the CCS related to specific energy by mechanical efficiency E, deriving unconfined compressive strength (UCS) from the CCS, monitoring placement of a horizontal well bore in a drilling operation based on the UCS and CCS.

In a further embodiment, a machine-readable storage medium has stored thereon a computer program for predicting rock strength for use in horizontal well drilling. The computer program includes a routine of set instructions for causing the machine to use drilling and bit data to calculate mechanical specific energy (MSE), drilling, mud and bit data to calculate hydraulic specific energy (HSE), obtain a specific energy value from the MSE and HSE, calculate confined compressive strength (CCS) from the specific energy, the CCS related to specific energy by mechanical efficiency E, derive unconfined compressive strength (UCS) from the CCS, and monitor placement of a horizontal well bore in a drilling operation based on the UCS and CCS.

Any combinations of the various embodiments and implementations disclosed herein can be used in a further embodiment, consistent with the disclosure. These and other aspects and features can be appreciated from the following description of certain embodiments presented herein in accordance with the disclosure and the accompanying drawings and claims.

Embodiments of the present disclosure will now be described in detail with reference to the accompanying drawing figures. Like elements in the various figures may be denoted by like reference numerals. Further, in the following detailed description, specific details are set forth in order to provide a more thorough understanding of the claimed subject matter. However, it will be apparent to one of ordinary skill in the art that the embodiments disclosed herein may be practiced without these specific details, or with details that are not described herein in the interest of clarity. Thus in some instances, well-known features have not been described in detail to avoid unnecessarily complicating the description. Additionally, it will be apparent to one of ordinary skill in the art that the scale of the elements presented in the accompanying drawing figures may vary without departing from the scope of the present disclosure.

Embodiments in accordance with the present disclosure generally relate to geosteering a wellbore using predicted confined compressive strength (CCS) and unconfined compressive strength (UCS) of subterranean formations.

In drilling subterranean formations, the drill bit applies mechanical forces on the formation rock at the bottom of the well. The mechanical forces include the compressive forces in the direction of drilling and rotational forces tangent to the direction of drilling. At the same time, pumping of drilling fluid into the wellbore at a high rate applies hydraulic forces on the rock. The energy required to destroy the formation rock comes from both the mechanical forces applied by the drill bit and the hydraulic forces exerted by the drilling fluid. The energy required to remove a unit volume of rock is defined as specific energy, which includes the mechanical specific energy and the hydraulic specific energy. Equation 1 and Equation 2 below define the mechanical specific energy and hydraulic specific energy, respectively.

Pressure drop across the bit is calculated with Equation 3

Combining Equation 1 and Equation 2, specific energy considering both mechanical energy input and hydraulic energy input is defined in Equation 4.

At perfect mechanical efficiency, the energy input into drilling is assumed to be the same as the CCS of the rock. However, the mechanical efficiency is usually much less than perfect, we therefore use a mechanical efficiency Eto relate the specific energy to CCS as shown in Equation 5.

Eis dependent on the formation rock, bit type, bit wear conditions and other factors, it typically varies between 0.1 and 0.4 for polycrystalline diamond compact (PDC) drill bits. However, the value for a certain formation needs to be evaluated with the actual data.

The determination of mechanical efficiency for a specific bit type drilling in a specific formation requires predicting ROP and comparing the predicted ROP with measured ROP in existing wells. With all the required input data from drilling and geomechanical analysis, the predicted ROP will match the measured ROP when the correct mechanical efficiency is used in the prediction. The details of such analysis can be found in U.S. patent application Ser. No. 18/323,701, filed on May 25, 2023, the contents of which are incorporated herein by reference in their entirety.

Combining Equation 4 and Equation 5, CCS is related to the drilling parameters in Equation 6.

When Wis zero, which means the drill bit is off bottom, T and ROP will be zero. If drilling fluid is still pumped into the wellbore, the calculated CCS based on Equation 6 will be infinite. To correct for this behavior, a scaling factor is added to Equation 6 as shown in Equation 7.

Bit wear condition deteriorates as more formations are drilled with the same drill bit and ROP will decrease accordingly. To exclude such effect on the calculated CCS, ROP reduction due to bit wear is estimated with Equation 8.

The total footage drilled by a bit and the average dull grade are estimated based on data of same type of bit drilled in the same formation.

From Equation 8, ROP of a fresh bit can be derived in Equation 9.

ROP in Equation 7 is replaced with ROPto ensure consistency and Equation 10 is the final equation for CCS, with pressure drop across bit expanded.

CCS is a function of UCS, confining pressure and friction angle. UCS is a commonly used rock strength parameter and often measured on core plugs in the laboratories and calculated using well logs. To validate the calculation of CCS using drilling parameters, it is necessary to derive UCS from CCS so that UCS can be compared with either measurements or log-derived values.

With triaxial compression tests in the laboratory, both UCS and CCS are measured. CCS measured at confining pressures that are equivalent to the downhole confining pressure conditions is then related to the UCS. Friction angle is used in the relationship. Equation 11 below shows one example of the relationship between CCS and the input data of confining pressure, UCS and friction angle.

With the calculated CCS from drilling parameters, UCS can be derived using Equation 12.

The confining pressure condition at the bottom of a drilling well depends on the permeability of the formation rock. For permeable rock, the confining pressure is the same as the differential pressure between well pressure and formation pore pressure as shown in Equation 13.

In impermeable formations, the confining pressure at the bottom of a drilling well is also a function of the in-situ stress in the direction of the wellbore. Equation 14 shows the equation for confining pressure in such formations.

is a flow diagram of a methodfor predicting rock strength that can be used in horizontal well drilling operations in accordance with certain embodiments. The methodincludes using drilling and bit datato calculate mechanical specific energy (MSE) at. This can be performed in accordance with Equation 1 above, wherein: