how to plot a curve fitting on a semilog

im trying to plot a curve fitting on a semi log graph to get its slope this is what ive tried

pws=[1150 1794 1823 1850 1876 1890 1910];
t=[0 2 4 8 16 24 48];
tp=3*24;
for i=2:length(pws)
horn(i)=((t(i)+tp)/(t(i)))
end
semilogx(horn,pws,'O-')
m=polyfit(horn,pws,1)
m(1)
k=((162.6*q*b*u)/(m(1)*h))
set(gca,'YLim',[1780 1960])
hold on
mm=polyval(m,horn)
semilogx(horn,mm,'-r');

but when i try it all i get is that

[Walter Roberson]

Your variable q is not defined in this code.

[mohamed]

q=500;
pwf=1150;
tp=3*24;
h=22;
phi=0.2;
rw=0.3;
b=1.13;
ct=20e-6;
u=1;

Answer 1

[Walter Roberson]

You are using horn as your base variable to fit against, but horn is not monotonic. If you

plot(horn,pws)

you can see that clearly.

semilogx(horn,pws) does not show how bad the graph is because horn(1) is 0 (because you did not assign any value to horn(1)), and log(0) is -infinity, so the first data point is not drawn on the semilogx plot.

[mohamed]

ok after i tried deleting the first element in the vector pws and horn and it helped a little but the generated best fit is still wrong here it is

the best fit curve is suppose to cut the y axis at 1950 and have a slope of 99