Step 1: Recall the purpose of the Bourdet method:
Bourdet introduced plotting the derivative of pressure change alongside the pressure change itself, both on log log axes, because the derivative is far more sensitive to changes in flow regime than the pressure change curve alone, making transitions between wellbore storage, transition, and radial flow easy to spot by eye.
Step 2: Evaluate option A by comparing plot types:
On a Cartesian or semi log plot, the wide range of pressure and time values recorded during a well test would compress early time behavior and stretch late time behavior unevenly. A log log plot instead spreads early and late time data proportionally, letting characteristic slopes, such as unit slope for wellbore storage, appear as straight lines. This is why the Bourdet plot is always drawn log log, confirming option A.
Step 3: Evaluate option B by considering what makes the type curves universal:
If the horizontal axis were simply $t_D$, a separate type curve chart would be needed for every possible wellbore storage value, which is impractical. By instead using the ratio $t_D/C_D$ on the horizontal axis, Bourdet collapsed the storage dependence into the axis itself, leaving only the parameter $C_D e^{2s}$ to distinguish curves. Because the option states the axis is $t_D$ alone rather than this ratio, option B is false.
Step 4: Evaluate option C using the physics of radial flow:
Once wellbore storage and near wellbore skin effects have fully dissipated, the reservoir behaves as if flow is radial and infinite acting, and the dimensionless pressure derivative approaches the constant value of one half independent of how much storage or skin the well originally had. Because every curve in the family reaches this same flat trend in the middle time region, the curves visibly merge there, confirming option C.
Step 5: Evaluate option D by recalling what is actually matched:
Type curve matching relies on lining up both the measured pressure change and the measured derivative against the corresponding pair of curves on the chart, so the vertical axis must carry both $P_D$ and its derivative together, not $P_D$ alone. This dual plotting is precisely what makes the Bourdet method more diagnostic than the older pressure only type curves, so option D is false.
Final Answer:
\[ \boxed{\text{Options A and C are correct}} \]